diff --git a/setup.py b/setup.py
index ad6f85e..e911d13 100644
--- a/setup.py
+++ b/setup.py
@@ -11,7 +11,7 @@ VERSION_FILE = os.path.join(BASE_DIR, 'stix2', 'version.py')
 def get_version():
     with open(VERSION_FILE) as f:
         for line in f.readlines():
-            if line.startswith("__version__"):
+            if line.startswith('__version__'):
                 version = line.split()[-1].strip('"')
                 return version
         raise AttributeError("Package does not have a __version__")
diff --git a/stix2/__init__.py b/stix2/__init__.py
index 23f6e8b..d867d19 100644
--- a/stix2/__init__.py
+++ b/stix2/__init__.py
@@ -70,4 +70,4 @@ from .version import __version__
 
 _collect_stix2_obj_maps()
 
-DEFAULT_VERSION = "2.1"  # Default version will always be the latest STIX 2.X version
+DEFAULT_VERSION = '2.1'  # Default version will always be the latest STIX 2.X version
diff --git a/stix2/base.py b/stix2/base.py
index c26f60a..4dbcd67 100644
--- a/stix2/base.py
+++ b/stix2/base.py
@@ -104,11 +104,11 @@ class _STIXBase(collections.Mapping):
     def _check_at_least_one_property(self, list_of_properties=None):
         if not list_of_properties:
             list_of_properties = sorted(list(self.__class__._properties.keys()))
-            if "type" in list_of_properties:
-                list_of_properties.remove("type")
+            if 'type' in list_of_properties:
+                list_of_properties.remove('type')
         current_properties = self.properties_populated()
         list_of_properties_populated = set(list_of_properties).intersection(current_properties)
-        if list_of_properties and (not list_of_properties_populated or list_of_properties_populated == set(["extensions"])):
+        if list_of_properties and (not list_of_properties_populated or list_of_properties_populated == set(['extensions'])):
             raise AtLeastOnePropertyError(self.__class__, list_of_properties)
 
     def _check_properties_dependency(self, list_of_properties, list_of_dependent_properties):
@@ -121,8 +121,8 @@ class _STIXBase(collections.Mapping):
             raise DependentPropertiesError(self.__class__, failed_dependency_pairs)
 
     def _check_object_constraints(self):
-        for m in self.get("granular_markings", []):
-            validate(self, m.get("selectors"))
+        for m in self.get('granular_markings', []):
+            validate(self, m.get('selectors'))
 
     def __init__(self, allow_custom=False, **kwargs):
         cls = self.__class__
@@ -190,7 +190,7 @@ class _STIXBase(collections.Mapping):
         # usual behavior of this method reads an __init__-assigned attribute,
         # which would cause infinite recursion.  So this check disables all
         # attribute reads until the instance has been properly initialized.
-        unpickling = "_inner" not in self.__dict__
+        unpickling = '_inner' not in self.__dict__
         if not unpickling and name in self:
             return self.__getitem__(name)
         raise AttributeError("'%s' object has no attribute '%s'" %
@@ -206,8 +206,8 @@ class _STIXBase(collections.Mapping):
 
     def __repr__(self):
         props = [(k, self[k]) for k in self.object_properties() if self.get(k)]
-        return "{0}({1})".format(self.__class__.__name__,
-                                 ", ".join(["{0!s}={1!r}".format(k, v) for k, v in props]))
+        return '{0}({1})'.format(self.__class__.__name__,
+                                 ', '.join(['{0!s}={1!r}'.format(k, v) for k, v in props]))
 
     def __deepcopy__(self, memo):
         # Assume: we can ignore the memo argument, because no object will ever contain the same sub-object multiple times.
@@ -273,7 +273,7 @@ class _STIXBase(collections.Mapping):
             def sort_by(element):
                 return find_property_index(self, *element)
 
-            kwargs.update({'indent': 4, 'separators': (",", ": "), 'item_sort_key': sort_by})
+            kwargs.update({'indent': 4, 'separators': (',', ': '), 'item_sort_key': sort_by})
 
         if include_optional_defaults:
             return json.dumps(self, cls=STIXJSONIncludeOptionalDefaultsEncoder, **kwargs)
diff --git a/stix2/confidence/scales.py b/stix2/confidence/scales.py
index a630be9..f7b9453 100644
--- a/stix2/confidence/scales.py
+++ b/stix2/confidence/scales.py
@@ -31,13 +31,13 @@ def none_low_med_high_to_value(scale_value):
     Raises:
         ValueError: If `scale_value` is not within the accepted strings.
     """
-    if scale_value == "None":
+    if scale_value == 'None':
         return 0
-    elif scale_value == "Low":
+    elif scale_value == 'Low':
         return 15
-    elif scale_value == "Med":
+    elif scale_value == 'Med':
         return 50
-    elif scale_value == "High":
+    elif scale_value == 'High':
         return 85
     else:
         raise ValueError("STIX Confidence value cannot be determined for %s" % scale_value)
@@ -69,13 +69,13 @@ def value_to_none_low_medium_high(confidence_value):
 
     """
     if confidence_value == 0:
-        return "None"
+        return 'None'
     elif 29 >= confidence_value >= 1:
-        return "Low"
+        return 'Low'
     elif 69 >= confidence_value >= 30:
-        return "Med"
+        return 'Med'
     elif 100 >= confidence_value >= 70:
-        return "High"
+        return 'High'
     else:
         raise ValueError("Range of values out of bounds: %s" % confidence_value)
 
@@ -114,27 +114,27 @@ def zero_ten_to_value(scale_value):
         ValueError: If `scale_value` is not within the accepted strings.
 
     """
-    if scale_value == "0":
+    if scale_value == '0':
         return 0
-    elif scale_value == "1":
+    elif scale_value == '1':
         return 10
-    elif scale_value == "2":
+    elif scale_value == '2':
         return 20
-    elif scale_value == "3":
+    elif scale_value == '3':
         return 30
-    elif scale_value == "4":
+    elif scale_value == '4':
         return 40
-    elif scale_value == "5":
+    elif scale_value == '5':
         return 50
-    elif scale_value == "6":
+    elif scale_value == '6':
         return 60
-    elif scale_value == "7":
+    elif scale_value == '7':
         return 70
-    elif scale_value == "8":
+    elif scale_value == '8':
         return 80
-    elif scale_value == "9":
+    elif scale_value == '9':
         return 90
-    elif scale_value == "10":
+    elif scale_value == '10':
         return 100
     else:
         raise ValueError("STIX Confidence value cannot be determined for %s" % scale_value)
@@ -173,27 +173,27 @@ def value_to_zero_ten(confidence_value):
 
     """
     if 4 >= confidence_value >= 0:
-        return "0"
+        return '0'
     elif 14 >= confidence_value >= 5:
-        return "1"
+        return '1'
     elif 24 >= confidence_value >= 15:
-        return "2"
+        return '2'
     elif 34 >= confidence_value >= 25:
-        return "3"
+        return '3'
     elif 44 >= confidence_value >= 35:
-        return "4"
+        return '4'
     elif 54 >= confidence_value >= 45:
-        return "5"
+        return '5'
     elif 64 >= confidence_value >= 55:
-        return "6"
+        return '6'
     elif 74 >= confidence_value >= 65:
-        return "7"
+        return '7'
     elif 84 >= confidence_value >= 75:
-        return "8"
+        return '8'
     elif 94 >= confidence_value >= 85:
-        return "9"
+        return '9'
     elif 100 >= confidence_value >= 95:
-        return "10"
+        return '10'
     else:
         raise ValueError("Range of values out of bounds: %s" % confidence_value)
 
@@ -229,17 +229,17 @@ def admiralty_credibility_to_value(scale_value):
         ValueError: If `scale_value` is not within the accepted strings.
 
     """
-    if scale_value == "6 - Truth cannot be judged":
-        raise ValueError("STIX Confidence value cannot be determined for %s" % scale_value)  # TODO: What happens here?
-    elif scale_value == "5 - Improbable":
+    if scale_value == '6 - Truth cannot be judged':
+        raise ValueError("STIX Confidence value cannot be determined for %s" % scale_value)
+    elif scale_value == '5 - Improbable':
         return 10
-    elif scale_value == "4 - Doubtful":
+    elif scale_value == '4 - Doubtful':
         return 30
-    elif scale_value == "3 - Possibly True":
+    elif scale_value == '3 - Possibly True':
         return 50
-    elif scale_value == "2 - Probably True":
+    elif scale_value == '2 - Probably True':
         return 70
-    elif scale_value == "1 - Confirmed by other sources":
+    elif scale_value == '1 - Confirmed by other sources':
         return 90
     else:
         raise ValueError("STIX Confidence value cannot be determined for %s" % scale_value)
@@ -272,17 +272,16 @@ def value_to_admiralty_credibility(confidence_value):
         ValueError: If `confidence_value` is out of bounds.
 
     """
-    # TODO: Case "6 - Truth cannot be judged"
     if 19 >= confidence_value >= 0:
-        return "5 - Improbable"
+        return '5 - Improbable'
     elif 39 >= confidence_value >= 20:
-        return "4 - Doubtful"
+        return '4 - Doubtful'
     elif 59 >= confidence_value >= 40:
-        return "3 - Possibly True"
+        return '3 - Possibly True'
     elif 79 >= confidence_value >= 60:
-        return "2 - Probably True"
+        return '2 - Probably True'
     elif 100 >= confidence_value >= 80:
-        return "1 - Confirmed by other sources"
+        return '1 - Confirmed by other sources'
     else:
         raise ValueError("Range of values out of bounds: %s" % confidence_value)
 
@@ -320,19 +319,19 @@ def wep_to_value(scale_value):
         ValueError: If `scale_value` is not within the accepted strings.
 
     """
-    if scale_value == "Impossible":
+    if scale_value == 'Impossible':
         return 0
-    elif scale_value == "Highly Unlikely/Almost Certainly Not":
+    elif scale_value == 'Highly Unlikely/Almost Certainly Not':
         return 10
-    elif scale_value == "Unlikely/Probably Not":
+    elif scale_value == 'Unlikely/Probably Not':
         return 30
-    elif scale_value == "Even Chance":
+    elif scale_value == 'Even Chance':
         return 50
-    elif scale_value == "Likely/Probable":
+    elif scale_value == 'Likely/Probable':
         return 70
-    elif scale_value == "Highly likely/Almost Certain":
+    elif scale_value == 'Highly likely/Almost Certain':
         return 90
-    elif scale_value == "Certain":
+    elif scale_value == 'Certain':
         return 100
     else:
         raise ValueError("STIX Confidence value cannot be determined for %s" % scale_value)
@@ -367,19 +366,19 @@ def value_to_wep(confidence_value):
 
     """
     if confidence_value == 0:
-        return "Impossible"
+        return 'Impossible'
     elif 19 >= confidence_value >= 1:
-        return "Highly Unlikely/Almost Certainly Not"
+        return 'Highly Unlikely/Almost Certainly Not'
     elif 39 >= confidence_value >= 20:
-        return "Unlikely/Probably Not"
+        return 'Unlikely/Probably Not'
     elif 59 >= confidence_value >= 40:
-        return "Even Chance"
+        return 'Even Chance'
     elif 79 >= confidence_value >= 60:
-        return "Likely/Probable"
+        return 'Likely/Probable'
     elif 99 >= confidence_value >= 80:
-        return "Highly likely/Almost Certain"
+        return 'Highly likely/Almost Certain'
     elif confidence_value == 100:
-        return "Certain"
+        return 'Certain'
     else:
         raise ValueError("Range of values out of bounds: %s" % confidence_value)
 
@@ -417,19 +416,19 @@ def dni_to_value(scale_value):
         ValueError: If `scale_value` is not within the accepted strings.
 
     """
-    if scale_value == "Almost No Chance / Remote":
+    if scale_value == 'Almost No Chance / Remote':
         return 5
-    elif scale_value == "Very Unlikely / Highly Improbable":
+    elif scale_value == 'Very Unlikely / Highly Improbable':
         return 15
-    elif scale_value == "Unlikely / Improbable":
+    elif scale_value == 'Unlikely / Improbable':
         return 30
-    elif scale_value == "Roughly Even Change / Roughly Even Odds":
+    elif scale_value == 'Roughly Even Change / Roughly Even Odds':
         return 50
-    elif scale_value == "Likely / Probable":
+    elif scale_value == 'Likely / Probable':
         return 70
-    elif scale_value == "Very Likely / Highly Probable":
+    elif scale_value == 'Very Likely / Highly Probable':
         return 85
-    elif scale_value == "Almost Certain / Nearly Certain":
+    elif scale_value == 'Almost Certain / Nearly Certain':
         return 95
     else:
         raise ValueError("STIX Confidence value cannot be determined for %s" % scale_value)
@@ -464,18 +463,18 @@ def value_to_dni(confidence_value):
 
     """
     if 9 >= confidence_value >= 0:
-        return "Almost No Chance / Remote"
+        return 'Almost No Chance / Remote'
     elif 19 >= confidence_value >= 10:
-        return "Very Unlikely / Highly Improbable"
+        return 'Very Unlikely / Highly Improbable'
     elif 39 >= confidence_value >= 20:
-        return "Unlikely / Improbable"
+        return 'Unlikely / Improbable'
     elif 59 >= confidence_value >= 40:
-        return "Roughly Even Change / Roughly Even Odds"
+        return 'Roughly Even Change / Roughly Even Odds'
     elif 79 >= confidence_value >= 60:
-        return "Likely / Probable"
+        return 'Likely / Probable'
     elif 89 >= confidence_value >= 80:
-        return "Very Likely / Highly Probable"
+        return 'Very Likely / Highly Probable'
     elif 100 >= confidence_value >= 90:
-        return "Almost Certain / Nearly Certain"
+        return 'Almost Certain / Nearly Certain'
     else:
         raise ValueError("Range of values out of bounds: %s" % confidence_value)
diff --git a/stix2/core.py b/stix2/core.py
index 934dfc4..3ee0f6b 100644
--- a/stix2/core.py
+++ b/stix2/core.py
@@ -76,11 +76,11 @@ def dict_to_stix2(stix_dict, allow_custom=False, version=None):
     if 'type' not in stix_dict:
         raise ParseError("Can't parse object with no 'type' property: %s" % str(stix_dict))
 
-    if "spec_version" in stix_dict:
+    if 'spec_version' in stix_dict:
         # For STIX 2.0, applies to bundles only.
         # For STIX 2.1+, applies to SDOs, SROs, and markings only.
-        v = 'v' + stix_dict["spec_version"].replace('.', '')
-    elif stix_dict["type"] == "bundle":
+        v = 'v' + stix_dict['spec_version'].replace('.', '')
+    elif stix_dict['type'] == 'bundle':
         # bundles without spec_version are ambiguous.
         if version:
             v = 'v' + version.replace('.', '')
diff --git a/stix2/datastore/__init__.py b/stix2/datastore/__init__.py
index c2963e2..376afc3 100644
--- a/stix2/datastore/__init__.py
+++ b/stix2/datastore/__init__.py
@@ -83,7 +83,8 @@ class DataStoreMixin(object):
         try:
             return self.source.get(*args, **kwargs)
         except AttributeError:
-            raise AttributeError('%s has no data source to query' % self.__class__.__name__)
+            msg = "%s has no data source to query"
+            raise AttributeError(msg % self.__class__.__name__)
 
     def all_versions(self, *args, **kwargs):
         """Retrieve all versions of a single STIX object by ID.
@@ -100,7 +101,8 @@ class DataStoreMixin(object):
         try:
             return self.source.all_versions(*args, **kwargs)
         except AttributeError:
-            raise AttributeError('%s has no data source to query' % self.__class__.__name__)
+            msg = "%s has no data source to query"
+            raise AttributeError(msg % self.__class__.__name__)
 
     def query(self, *args, **kwargs):
         """Retrieve STIX objects matching a set of filters.
@@ -118,7 +120,8 @@ class DataStoreMixin(object):
         try:
             return self.source.query(*args, **kwargs)
         except AttributeError:
-            raise AttributeError('%s has no data source to query' % self.__class__.__name__)
+            msg = "%s has no data source to query"
+            raise AttributeError(msg % self.__class__.__name__)
 
     def creator_of(self, *args, **kwargs):
         """Retrieve the Identity refered to by the object's `created_by_ref`.
@@ -137,7 +140,8 @@ class DataStoreMixin(object):
         try:
             return self.source.creator_of(*args, **kwargs)
         except AttributeError:
-            raise AttributeError('%s has no data source to query' % self.__class__.__name__)
+            msg = "%s has no data source to query"
+            raise AttributeError(msg % self.__class__.__name__)
 
     def relationships(self, *args, **kwargs):
         """Retrieve Relationships involving the given STIX object.
@@ -163,7 +167,8 @@ class DataStoreMixin(object):
         try:
             return self.source.relationships(*args, **kwargs)
         except AttributeError:
-            raise AttributeError('%s has no data source to query' % self.__class__.__name__)
+            msg = "%s has no data source to query"
+            raise AttributeError(msg % self.__class__.__name__)
 
     def related_to(self, *args, **kwargs):
         """Retrieve STIX Objects that have a Relationship involving the given
@@ -193,7 +198,8 @@ class DataStoreMixin(object):
         try:
             return self.source.related_to(*args, **kwargs)
         except AttributeError:
-            raise AttributeError('%s has no data source to query' % self.__class__.__name__)
+            msg = "%s has no data source to query"
+            raise AttributeError(msg % self.__class__.__name__)
 
     def add(self, *args, **kwargs):
         """Method for storing STIX objects.
@@ -208,7 +214,8 @@ class DataStoreMixin(object):
         try:
             return self.sink.add(*args, **kwargs)
         except AttributeError:
-            raise AttributeError('%s has no data sink to put objects in' % self.__class__.__name__)
+            msg = "%s has no data sink to put objects in"
+            raise AttributeError(msg % self.__class__.__name__)
 
 
 class DataSink(with_metaclass(ABCMeta)):
@@ -457,7 +464,7 @@ class CompositeDataSource(DataSource):
 
         """
         if not self.has_data_sources():
-            raise AttributeError('CompositeDataSource has no data sources')
+            raise AttributeError("CompositeDataSource has no data sources")
 
         all_data = []
         all_filters = FilterSet()
@@ -504,7 +511,7 @@ class CompositeDataSource(DataSource):
 
         """
         if not self.has_data_sources():
-            raise AttributeError('CompositeDataSource has no data sources')
+            raise AttributeError("CompositeDataSource has no data sources")
 
         all_data = []
         all_filters = FilterSet()
@@ -543,7 +550,7 @@ class CompositeDataSource(DataSource):
 
         """
         if not self.has_data_sources():
-            raise AttributeError('CompositeDataSource has no data sources')
+            raise AttributeError("CompositeDataSource has no data sources")
 
         if not query:
             # don't mess with the query (i.e. deduplicate, as that's done
@@ -594,7 +601,7 @@ class CompositeDataSource(DataSource):
 
         """
         if not self.has_data_sources():
-            raise AttributeError('CompositeDataSource has no data sources')
+            raise AttributeError("CompositeDataSource has no data sources")
 
         results = []
         for ds in self.data_sources:
@@ -634,7 +641,7 @@ class CompositeDataSource(DataSource):
 
         """
         if not self.has_data_sources():
-            raise AttributeError('CompositeDataSource has no data sources')
+            raise AttributeError("CompositeDataSource has no data sources")
 
         results = []
         for ds in self.data_sources:
diff --git a/stix2/datastore/filesystem.py b/stix2/datastore/filesystem.py
index d508dd8..124fa9a 100644
--- a/stix2/datastore/filesystem.py
+++ b/stix2/datastore/filesystem.py
@@ -1,7 +1,4 @@
-"""
-Python STIX 2.0 FileSystem Source/Sink
-
-"""
+"""Python STIX 2.0 FileSystem Source/Sink"""
 
 import json
 import os
@@ -78,7 +75,7 @@ class FileSystemSink(DataSink):
     def _check_path_and_write(self, stix_obj):
         """Write the given STIX object to a file in the STIX file directory.
         """
-        path = os.path.join(self._stix_dir, stix_obj["type"], stix_obj["id"] + ".json")
+        path = os.path.join(self._stix_dir, stix_obj['type'], stix_obj['id'] + '.json')
 
         if not os.path.exists(os.path.dirname(path)):
             os.makedirs(os.path.dirname(path))
@@ -86,7 +83,7 @@ class FileSystemSink(DataSink):
         if self.bundlify:
             stix_obj = Bundle(stix_obj, allow_custom=self.allow_custom)
 
-        with open(path, "w") as f:
+        with open(path, 'w') as f:
             f.write(str(stix_obj))
 
     def add(self, stix_data=None, version=None):
@@ -112,9 +109,9 @@ class FileSystemSink(DataSink):
 
         elif isinstance(stix_data, (str, dict)):
             stix_data = parse(stix_data, allow_custom=self.allow_custom, version=version)
-            if stix_data["type"] == "bundle":
+            if stix_data['type'] == 'bundle':
                 # extract STIX objects
-                for stix_obj in stix_data.get("objects", []):
+                for stix_obj in stix_data.get('objects', []):
                     self.add(stix_obj, version=version)
             else:
                 # adding json-formatted STIX
@@ -122,7 +119,7 @@ class FileSystemSink(DataSink):
 
         elif isinstance(stix_data, Bundle):
             # recursively add individual STIX objects
-            for stix_obj in stix_data.get("objects", []):
+            for stix_obj in stix_data.get('objects', []):
                 self.add(stix_obj, version=version)
 
         elif isinstance(stix_data, list):
@@ -177,7 +174,7 @@ class FileSystemSource(DataSource):
                 a python STIX object and then returned
 
         """
-        query = [Filter("id", "=", stix_id)]
+        query = [Filter('id', '=', stix_id)]
 
         all_data = self.query(query=query, version=version, _composite_filters=_composite_filters)
 
@@ -252,12 +249,12 @@ class FileSystemSource(DataSource):
         # the corresponding subdirectories as well
         include_paths = []
         declude_paths = []
-        if "type" in [filter.property for filter in file_filters]:
+        if 'type' in [filter.property for filter in file_filters]:
             for filter in file_filters:
-                if filter.property == "type":
-                    if filter.op == "=":
+                if filter.property == 'type':
+                    if filter.op == '=':
                         include_paths.append(os.path.join(self._stix_dir, filter.value))
-                    elif filter.op == "!=":
+                    elif filter.op == '!=':
                         declude_paths.append(os.path.join(self._stix_dir, filter.value))
         else:
             # have to walk entire STIX directory
@@ -281,9 +278,9 @@ class FileSystemSource(DataSource):
 
         # grab stix object ID as well - if present in filters, as
         # may forgo the loading of STIX content into memory
-        if "id" in [filter.property for filter in file_filters]:
+        if 'id' in [filter.property for filter in file_filters]:
             for filter in file_filters:
-                if filter.property == "id" and filter.op == "=":
+                if filter.property == 'id' and filter.op == '=':
                     id_ = filter.value
                     break
             else:
@@ -295,21 +292,21 @@ class FileSystemSource(DataSource):
         for path in include_paths:
             for root, dirs, files in os.walk(path):
                 for file_ in files:
-                    if not file_.endswith(".json"):
+                    if not file_.endswith('.json'):
                         # skip non '.json' files as more likely to be random non-STIX files
                         continue
 
-                    if not id_ or id_ == file_.split(".")[0]:
+                    if not id_ or id_ == file_.split('.')[0]:
                         # have to load into memory regardless to evaluate other filters
                         try:
                             stix_obj = json.load(open(os.path.join(root, file_)))
 
-                            if stix_obj["type"] == "bundle":
-                                stix_obj = stix_obj["objects"][0]
+                            if stix_obj['type'] == 'bundle':
+                                stix_obj = stix_obj['objects'][0]
 
                             # naive STIX type checking
-                            stix_obj["type"]
-                            stix_obj["id"]
+                            stix_obj['type']
+                            stix_obj['id']
 
                         except (ValueError, KeyError):  # likely not a JSON file
                             raise TypeError("STIX JSON object at '{0}' could either not be parsed to "
@@ -339,6 +336,6 @@ class FileSystemSource(DataSource):
         """
         file_filters = []
         for filter_ in query:
-            if filter_.property == "id" or filter_.property == "type":
+            if filter_.property == 'id' or filter_.property == 'type':
                 file_filters.append(filter_)
         return file_filters
diff --git a/stix2/datastore/filters.py b/stix2/datastore/filters.py
index a32b14a..766351c 100644
--- a/stix2/datastore/filters.py
+++ b/stix2/datastore/filters.py
@@ -1,7 +1,4 @@
-"""
-Filters for Python STIX 2.0 DataSources, DataSinks, DataStores
-
-"""
+"""Filters for Python STIX 2.0 DataSources, DataSinks, DataStores"""
 
 import collections
 from datetime import datetime
@@ -40,14 +37,14 @@ def _check_filter_components(prop, op, value):
         # check filter value type is supported
         raise TypeError("Filter value of '%s' is not supported. The type must be a Python immutable type or dictionary" % type(value))
 
-    if prop == "type" and "_" in value:
+    if prop == 'type' and '_' in value:
         # check filter where the property is type, value (type name) cannot have underscores
         raise ValueError("Filter for property 'type' cannot have its value '%s' include underscores" % value)
 
     return True
 
 
-class Filter(collections.namedtuple("Filter", ['property', 'op', 'value'])):
+class Filter(collections.namedtuple('Filter', ['property', 'op', 'value'])):
     """STIX 2 filters that support the querying functionality of STIX 2
     DataStores and DataSources.
 
@@ -94,19 +91,19 @@ class Filter(collections.namedtuple("Filter", ['property', 'op', 'value'])):
             # but will put here for now
             stix_obj_property = format_datetime(stix_obj_property)
 
-        if self.op == "=":
+        if self.op == '=':
             return stix_obj_property == self.value
-        elif self.op == "!=":
+        elif self.op == '!=':
             return stix_obj_property != self.value
-        elif self.op == "in":
+        elif self.op == 'in':
             return stix_obj_property in self.value
-        elif self.op == ">":
+        elif self.op == '>':
             return stix_obj_property > self.value
-        elif self.op == "<":
+        elif self.op == '<':
             return stix_obj_property < self.value
-        elif self.op == ">=":
+        elif self.op == '>=':
             return stix_obj_property >= self.value
-        elif self.op == "<=":
+        elif self.op == '<=':
             return stix_obj_property <= self.value
         else:
             raise ValueError("Filter operator: {0} not supported for specified property: {1}".format(self.op, self.property))
@@ -153,7 +150,7 @@ def _check_filter(filter_, stix_obj):
     """
     # For properties like granular_markings and external_references
     # need to extract the first property from the string.
-    prop = filter_.property.split(".")[0]
+    prop = filter_.property.split('.')[0]
 
     if prop not in stix_obj.keys():
         # check filter "property" is in STIX object - if cant be
@@ -161,9 +158,9 @@ def _check_filter(filter_, stix_obj):
         # (i.e. did not make it through the filter)
         return False
 
-    if "." in filter_.property:
+    if '.' in filter_.property:
         # Check embedded properties, from e.g. granular_markings or external_references
-        sub_property = filter_.property.split(".", 1)[1]
+        sub_property = filter_.property.split('.', 1)[1]
         sub_filter = filter_._replace(property=sub_property)
 
         if isinstance(stix_obj[prop], list):
diff --git a/stix2/datastore/memory.py b/stix2/datastore/memory.py
index adb0875..13f5452 100644
--- a/stix2/datastore/memory.py
+++ b/stix2/datastore/memory.py
@@ -36,16 +36,16 @@ def _add(store, stix_data=None, version=None):
     """
     if isinstance(stix_data, _STIXBase):
         # adding a python STIX object
-        store._data[stix_data["id"]] = stix_data
+        store._data[stix_data['id']] = stix_data
 
     elif isinstance(stix_data, dict):
-        if stix_data["type"] == "bundle":
+        if stix_data['type'] == 'bundle':
             # adding a json bundle - so just grab STIX objects
-            for stix_obj in stix_data.get("objects", []):
+            for stix_obj in stix_data.get('objects', []):
                 _add(store, stix_obj, version=version)
         else:
             # adding a json STIX object
-            store._data[stix_data["id"]] = stix_data
+            store._data[stix_data['id']] = stix_data
 
     elif isinstance(stix_data, list):
         # STIX objects are in a list- recurse on each object
@@ -156,7 +156,7 @@ class MemorySink(DataSink):
 
         if not os.path.exists(os.path.dirname(file_path)):
             os.makedirs(os.path.dirname(file_path))
-        with open(file_path, "w") as f:
+        with open(file_path, 'w') as f:
             f.write(str(Bundle(list(self._data.values()), allow_custom=self.allow_custom)))
     save_to_file.__doc__ = MemoryStore.save_to_file.__doc__
 
@@ -217,7 +217,7 @@ class MemorySource(DataSource):
             return stix_obj
 
         # if there are filters from the composite level, process full query
-        query = [Filter("id", "=", stix_id)]
+        query = [Filter('id', '=', stix_id)]
 
         all_data = self.query(query=query, _composite_filters=_composite_filters)
 
@@ -283,10 +283,10 @@ class MemorySource(DataSource):
         return all_data
 
     def load_from_file(self, file_path, version=None):
-        stix_data = json.load(open(os.path.abspath(file_path), "r"))
+        stix_data = json.load(open(os.path.abspath(file_path), 'r'))
 
-        if stix_data["type"] == "bundle":
-            for stix_obj in stix_data["objects"]:
+        if stix_data['type'] == 'bundle':
+            for stix_obj in stix_data['objects']:
                 _add(self, stix_data=parse(stix_obj, allow_custom=self.allow_custom))
         else:
             _add(self, stix_data=parse(stix_data, allow_custom=self.allow_custom, version=version))
diff --git a/stix2/datastore/taxii.py b/stix2/datastore/taxii.py
index 1a6bacc..2a8d5cb 100644
--- a/stix2/datastore/taxii.py
+++ b/stix2/datastore/taxii.py
@@ -1,6 +1,4 @@
-"""
-Python STIX 2.x TAXIICollectionStore
-"""
+"""Python STIX 2.x TAXIICollectionStore"""
 from requests.exceptions import HTTPError
 
 from stix2 import Bundle
@@ -89,17 +87,17 @@ class TAXIICollectionSink(DataSink):
         """
         if isinstance(stix_data, _STIXBase):
             # adding python STIX object
-            if stix_data["type"] == "bundle":
-                bundle = stix_data.serialize(encoding="utf-8")
+            if stix_data['type'] == 'bundle':
+                bundle = stix_data.serialize(encoding='utf-8')
             else:
-                bundle = Bundle(stix_data, allow_custom=self.allow_custom).serialize(encoding="utf-8")
+                bundle = Bundle(stix_data, allow_custom=self.allow_custom).serialize(encoding='utf-8')
 
         elif isinstance(stix_data, dict):
             # adding python dict (of either Bundle or STIX obj)
-            if stix_data["type"] == "bundle":
-                bundle = parse(stix_data, allow_custom=self.allow_custom, version=version).serialize(encoding="utf-8")
+            if stix_data['type'] == 'bundle':
+                bundle = parse(stix_data, allow_custom=self.allow_custom, version=version).serialize(encoding='utf-8')
             else:
-                bundle = Bundle(stix_data, allow_custom=self.allow_custom).serialize(encoding="utf-8")
+                bundle = Bundle(stix_data, allow_custom=self.allow_custom).serialize(encoding='utf-8')
 
         elif isinstance(stix_data, list):
             # adding list of something - recurse on each
@@ -110,10 +108,10 @@ class TAXIICollectionSink(DataSink):
         elif isinstance(stix_data, str):
             # adding json encoded string of STIX content
             stix_data = parse(stix_data, allow_custom=self.allow_custom, version=version)
-            if stix_data["type"] == "bundle":
-                bundle = stix_data.serialize(encoding="utf-8")
+            if stix_data['type'] == 'bundle':
+                bundle = stix_data.serialize(encoding='utf-8')
             else:
-                bundle = Bundle(stix_data, allow_custom=self.allow_custom).serialize(encoding="utf-8")
+                bundle = Bundle(stix_data, allow_custom=self.allow_custom).serialize(encoding='utf-8')
 
         else:
             raise TypeError("stix_data must be as STIX object(or list of),json formatted STIX (or list of), or a json formatted STIX bundle")
@@ -177,7 +175,7 @@ class TAXIICollectionSource(DataSource):
         # dont extract TAXII filters from query (to send to TAXII endpoint)
         # as directly retrieveing a STIX object by ID
         try:
-            stix_objs = self.collection.get_object(stix_id)["objects"]
+            stix_objs = self.collection.get_object(stix_id)['objects']
             stix_obj = list(apply_common_filters(stix_objs, query))
 
         except HTTPError as e:
@@ -214,8 +212,8 @@ class TAXIICollectionSource(DataSource):
         """
         # make query in TAXII query format since 'id' is TAXII field
         query = [
-            Filter("id", "=", stix_id),
-            Filter("version", "=", "all")
+            Filter('id', '=', stix_id),
+            Filter('version', '=', 'all')
         ]
 
         all_data = self.query(query=query, _composite_filters=_composite_filters)
@@ -264,7 +262,7 @@ class TAXIICollectionSource(DataSource):
 
         # query TAXII collection
         try:
-            all_data = self.collection.get_objects(**taxii_filters_dict)["objects"]
+            all_data = self.collection.get_objects(**taxii_filters_dict)['objects']
 
             # deduplicate data (before filtering as reduces wasted filtering)
             all_data = deduplicate(all_data)
diff --git a/stix2/markings/granular_markings.py b/stix2/markings/granular_markings.py
index 7c227d9..5f47f41 100644
--- a/stix2/markings/granular_markings.py
+++ b/stix2/markings/granular_markings.py
@@ -29,7 +29,7 @@ def get_markings(obj, selectors, inherited=False, descendants=False):
     selectors = utils.convert_to_list(selectors)
     utils.validate(obj, selectors)
 
-    granular_markings = obj.get("granular_markings", [])
+    granular_markings = obj.get('granular_markings', [])
 
     if not granular_markings:
         return []
@@ -38,11 +38,11 @@ def get_markings(obj, selectors, inherited=False, descendants=False):
 
     for marking in granular_markings:
         for user_selector in selectors:
-            for marking_selector in marking.get("selectors", []):
+            for marking_selector in marking.get('selectors', []):
                 if any([(user_selector == marking_selector),  # Catch explicit selectors.
                         (user_selector.startswith(marking_selector) and inherited),  # Catch inherited selectors.
                         (marking_selector.startswith(user_selector) and descendants)]):  # Catch descendants selectors
-                    refs = marking.get("marking_ref", [])
+                    refs = marking.get('marking_ref', [])
                     results.update([refs])
 
     return list(results)
@@ -93,7 +93,7 @@ def remove_markings(obj, marking, selectors):
     marking = utils.convert_to_marking_list(marking)
     utils.validate(obj, selectors)
 
-    granular_markings = obj.get("granular_markings")
+    granular_markings = obj.get('granular_markings')
 
     if not granular_markings:
         return obj
@@ -102,9 +102,9 @@ def remove_markings(obj, marking, selectors):
 
     to_remove = []
     for m in marking:
-        to_remove.append({"marking_ref": m, "selectors": selectors})
+        to_remove.append({'marking_ref': m, 'selectors': selectors})
 
-    remove = utils.build_granular_marking(to_remove).get("granular_markings")
+    remove = utils.build_granular_marking(to_remove).get('granular_markings')
 
     if not any(marking in granular_markings for marking in remove):
         raise exceptions.MarkingNotFoundError(obj, remove)
@@ -145,10 +145,10 @@ def add_markings(obj, marking, selectors):
 
     granular_marking = []
     for m in marking:
-        granular_marking.append({"marking_ref": m, "selectors": sorted(selectors)})
+        granular_marking.append({'marking_ref': m, 'selectors': sorted(selectors)})
 
-    if obj.get("granular_markings"):
-        granular_marking.extend(obj.get("granular_markings"))
+    if obj.get('granular_markings'):
+        granular_marking.extend(obj.get('granular_markings'))
 
     granular_marking = utils.expand_markings(granular_marking)
     granular_marking = utils.compress_markings(granular_marking)
@@ -176,7 +176,7 @@ def clear_markings(obj, selectors):
     selectors = utils.convert_to_list(selectors)
     utils.validate(obj, selectors)
 
-    granular_markings = obj.get("granular_markings")
+    granular_markings = obj.get('granular_markings')
 
     if not granular_markings:
         return obj
@@ -184,25 +184,25 @@ def clear_markings(obj, selectors):
     granular_markings = utils.expand_markings(granular_markings)
 
     sdo = utils.build_granular_marking(
-        [{"selectors": selectors, "marking_ref": "N/A"}]
+        [{'selectors': selectors, 'marking_ref': 'N/A'}]
     )
 
-    clear = sdo.get("granular_markings", [])
+    clear = sdo.get('granular_markings', [])
 
-    if not any(clear_selector in sdo_selectors.get("selectors", [])
+    if not any(clear_selector in sdo_selectors.get('selectors', [])
                for sdo_selectors in granular_markings
                for clear_marking in clear
-               for clear_selector in clear_marking.get("selectors", [])
+               for clear_selector in clear_marking.get('selectors', [])
                ):
         raise exceptions.MarkingNotFoundError(obj, clear)
 
     for granular_marking in granular_markings:
         for s in selectors:
-            if s in granular_marking.get("selectors", []):
-                marking_refs = granular_marking.get("marking_ref")
+            if s in granular_marking.get('selectors', []):
+                marking_refs = granular_marking.get('marking_ref')
 
                 if marking_refs:
-                    granular_marking["marking_ref"] = ""
+                    granular_marking['marking_ref'] = ''
 
     granular_markings = utils.compress_markings(granular_markings)
 
@@ -245,19 +245,19 @@ def is_marked(obj, marking=None, selectors=None, inherited=False, descendants=Fa
     marking = utils.convert_to_marking_list(marking)
     utils.validate(obj, selectors)
 
-    granular_markings = obj.get("granular_markings", [])
+    granular_markings = obj.get('granular_markings', [])
 
     marked = False
     markings = set()
 
     for granular_marking in granular_markings:
         for user_selector in selectors:
-            for marking_selector in granular_marking.get("selectors", []):
+            for marking_selector in granular_marking.get('selectors', []):
 
                 if any([(user_selector == marking_selector),  # Catch explicit selectors.
                         (user_selector.startswith(marking_selector) and inherited),  # Catch inherited selectors.
                         (marking_selector.startswith(user_selector) and descendants)]):  # Catch descendants selectors
-                    marking_ref = granular_marking.get("marking_ref", "")
+                    marking_ref = granular_marking.get('marking_ref', '')
 
                     if marking and any(x == marking_ref for x in marking):
                         markings.update([marking_ref])
diff --git a/stix2/markings/object_markings.py b/stix2/markings/object_markings.py
index a169fe3..53517be 100644
--- a/stix2/markings/object_markings.py
+++ b/stix2/markings/object_markings.py
@@ -18,7 +18,7 @@ def get_markings(obj):
             markings are present in `object_marking_refs`.
 
     """
-    return obj.get("object_marking_refs", [])
+    return obj.get('object_marking_refs', [])
 
 
 def add_markings(obj, marking):
@@ -35,7 +35,7 @@ def add_markings(obj, marking):
     """
     marking = utils.convert_to_marking_list(marking)
 
-    object_markings = set(obj.get("object_marking_refs", []) + marking)
+    object_markings = set(obj.get('object_marking_refs', []) + marking)
 
     return new_version(obj, object_marking_refs=list(object_markings), allow_custom=True)
 
@@ -59,12 +59,12 @@ def remove_markings(obj, marking):
     """
     marking = utils.convert_to_marking_list(marking)
 
-    object_markings = obj.get("object_marking_refs", [])
+    object_markings = obj.get('object_marking_refs', [])
 
     if not object_markings:
         return obj
 
-    if any(x not in obj["object_marking_refs"] for x in marking):
+    if any(x not in obj['object_marking_refs'] for x in marking):
         raise exceptions.MarkingNotFoundError(obj, marking)
 
     new_markings = [x for x in object_markings if x not in marking]
@@ -124,7 +124,7 @@ def is_marked(obj, marking=None):
 
     """
     marking = utils.convert_to_marking_list(marking)
-    object_markings = obj.get("object_marking_refs", [])
+    object_markings = obj.get('object_marking_refs', [])
 
     if marking:
         return any(x in object_markings for x in marking)
diff --git a/stix2/markings/utils.py b/stix2/markings/utils.py
index 429311b..4b0b64d 100644
--- a/stix2/markings/utils.py
+++ b/stix2/markings/utils.py
@@ -23,7 +23,7 @@ def _evaluate_expression(obj, selector):
 
     """
     for items, value in iterpath(obj):
-        path = ".".join(items)
+        path = '.'.join(items)
 
         if path == selector and value:
             return [value]
@@ -119,12 +119,12 @@ def compress_markings(granular_markings):
     map_ = collections.defaultdict(set)
 
     for granular_marking in granular_markings:
-        if granular_marking.get("marking_ref"):
-            map_[granular_marking.get("marking_ref")].update(granular_marking.get("selectors"))
+        if granular_marking.get('marking_ref'):
+            map_[granular_marking.get('marking_ref')].update(granular_marking.get('selectors'))
 
     compressed = \
         [
-            {"marking_ref": marking_ref, "selectors": sorted(selectors)}
+            {'marking_ref': marking_ref, 'selectors': sorted(selectors)}
             for marking_ref, selectors in six.iteritems(map_)
         ]
 
@@ -173,12 +173,12 @@ def expand_markings(granular_markings):
     expanded = []
 
     for marking in granular_markings:
-        selectors = marking.get("selectors")
-        marking_ref = marking.get("marking_ref")
+        selectors = marking.get('selectors')
+        marking_ref = marking.get('marking_ref')
 
         expanded.extend(
             [
-                {"marking_ref": marking_ref, "selectors": [selector]}
+                {'marking_ref': marking_ref, 'selectors': [selector]}
                 for selector in selectors
             ]
         )
@@ -189,7 +189,7 @@ def expand_markings(granular_markings):
 def build_granular_marking(granular_marking):
     """Return a dictionary with the required structure for a granular marking.
     """
-    return {"granular_markings": expand_markings(granular_marking)}
+    return {'granular_markings': expand_markings(granular_marking)}
 
 
 def iterpath(obj, path=None):
@@ -229,7 +229,7 @@ def iterpath(obj, path=None):
         elif isinstance(varobj, list):
 
             for item in varobj:
-                index = "[{0}]".format(varobj.index(item))
+                index = '[{0}]'.format(varobj.index(item))
                 path.append(index)
 
                 yield (path, item)
diff --git a/stix2/utils.py b/stix2/utils.py
index ab382b5..a5efb47 100644
--- a/stix2/utils.py
+++ b/stix2/utils.py
@@ -16,7 +16,7 @@ from .exceptions import (InvalidValueError, RevokeError,
 NOW = object()
 
 # STIX object properties that cannot be modified
-STIX_UNMOD_PROPERTIES = ["created", "created_by_ref", "id", "type"]
+STIX_UNMOD_PROPERTIES = ['created', 'created_by_ref', 'id', 'type']
 
 TYPE_REGEX = r'^\-?[a-z0-9]+(-[a-z0-9]+)*\-?$'
 
@@ -90,16 +90,16 @@ def format_datetime(dttm):
         zoned = pytz.utc.localize(dttm)
     else:
         zoned = dttm.astimezone(pytz.utc)
-    ts = zoned.strftime("%Y-%m-%dT%H:%M:%S")
-    ms = zoned.strftime("%f")
-    precision = getattr(dttm, "precision", None)
+    ts = zoned.strftime('%Y-%m-%dT%H:%M:%S')
+    ms = zoned.strftime('%f')
+    precision = getattr(dttm, 'precision', None)
     if precision == 'second':
         pass  # Already precise to the second
-    elif precision == "millisecond":
+    elif precision == 'millisecond':
         ts = ts + '.' + ms[:3]
     elif zoned.microsecond > 0:
-        ts = ts + '.' + ms.rstrip("0")
-    return ts + "Z"
+        ts = ts + '.' + ms.rstrip('0')
+    return ts + 'Z'
 
 
 def parse_into_datetime(value, precision=None):
@@ -250,11 +250,11 @@ def new_version(data, **kwargs):
     """
 
     if not isinstance(data, Mapping):
-        raise ValueError('cannot create new version of object of this type! '
-                         'Try a dictionary or instance of an SDO or SRO class.')
+        raise ValueError("cannot create new version of object of this type! "
+                         "Try a dictionary or instance of an SDO or SRO class.")
 
     unchangable_properties = []
-    if data.get("revoked"):
+    if data.get('revoked'):
         raise RevokeError("new_version")
     try:
         new_obj_inner = copy.deepcopy(data._inner)
@@ -292,10 +292,10 @@ def revoke(data):
         A new version of the object with ``revoked`` set to ``True``.
     """
     if not isinstance(data, Mapping):
-        raise ValueError('cannot revoke object of this type! Try a dictionary '
-                         'or instance of an SDO or SRO class.')
+        raise ValueError("cannot revoke object of this type! Try a dictionary "
+                         "or instance of an SDO or SRO class.")
 
-    if data.get("revoked"):
+    if data.get('revoked'):
         raise RevokeError("revoke")
     return new_version(data, revoked=True, allow_custom=True)
 
@@ -328,13 +328,13 @@ def remove_custom_stix(stix_obj):
         A new version of the object with any custom content removed
     """
 
-    if stix_obj["type"].startswith("x-"):
+    if stix_obj['type'].startswith('x-'):
         # if entire object is custom, discard
         return None
 
     custom_props = []
     for prop in stix_obj.items():
-        if prop[0].startswith("x_"):
+        if prop[0].startswith('x_'):
             # for every custom property, record it and set value to None
             # (so we can pass it to new_version() and it will be dropped)
             custom_props.append((prop[0], None))
@@ -351,7 +351,7 @@ def remove_custom_stix(stix_obj):
         # existing STIX object) and the "modified" property. We dont supply the
         # "modified" property so that new_version() creates a new datetime
         # value for this property
-        non_supplied_props = STIX_UNMOD_PROPERTIES + ["modified"]
+        non_supplied_props = STIX_UNMOD_PROPERTIES + ['modified']
 
         props = [(prop, stix_obj[prop]) for prop in stix_obj if prop not in non_supplied_props]
 
@@ -360,7 +360,7 @@ def remove_custom_stix(stix_obj):
 
         new_obj = new_version(stix_obj, **(dict(props)))
 
-        while parse_into_datetime(new_obj["modified"]) == parse_into_datetime(stix_obj["modified"]):
+        while parse_into_datetime(new_obj['modified']) == parse_into_datetime(stix_obj['modified']):
             # Prevents bug when fast computation allows multiple STIX object
             # versions to be created in single unit of time
             new_obj = new_version(stix_obj, **(dict(props)))
diff --git a/stix2/v20/bundle.py b/stix2/v20/bundle.py
index 05764b0..41da892 100644
--- a/stix2/v20/bundle.py
+++ b/stix2/v20/bundle.py
@@ -24,7 +24,7 @@ class STIXObjectProperty(Property):
             # validation here depend on the value of another property
             # (spec_version).  So this is a hack, and not technically spec-
             # compliant.
-            if "spec_version" in value:
+            if 'spec_version' in value:
                 raise ValueError("Spec version 2.0 bundles don't yet support "
                                  "containing objects of a different spec "
                                  "version.")
@@ -37,7 +37,7 @@ class STIXObjectProperty(Property):
             raise ValueError("This property may only contain a non-empty dictionary or object")
         if 'type' in dictified and dictified['type'] == 'bundle':
             raise ValueError('This property may not contain a Bundle object')
-        if "spec_version" in dictified:
+        if 'spec_version' in dictified:
             # See above comment regarding spec_version.
             raise ValueError("Spec version 2.0 bundles don't yet support "
                              "containing objects of a different spec version.")
@@ -53,13 +53,12 @@ class Bundle(_STIXBase):
     """
 
     _type = 'bundle'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
         # Not technically correct: STIX 2.0 spec doesn't say spec_version must
         # have this value, but it's all we support for now.
-        ('spec_version', StringProperty(fixed="2.0")),
+        ('spec_version', StringProperty(fixed='2.0')),
         ('objects', ListProperty(STIXObjectProperty)),
     ])
 
diff --git a/stix2/v20/common.py b/stix2/v20/common.py
index 47a464a..5e8e831 100644
--- a/stix2/v20/common.py
+++ b/stix2/v20/common.py
@@ -16,8 +16,7 @@ class ExternalReference(_STIXBase):
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709261>`__.
     """
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('source_name', StringProperty(required=True)),
         ('description', StringProperty()),
         ('url', StringProperty()),
@@ -27,7 +26,7 @@ class ExternalReference(_STIXBase):
 
     def _check_object_constraints(self):
         super(ExternalReference, self)._check_object_constraints()
-        self._check_at_least_one_property(["description", "external_id", "url"])
+        self._check_at_least_one_property(['description', 'external_id', 'url'])
 
 
 class KillChainPhase(_STIXBase):
@@ -36,8 +35,7 @@ class KillChainPhase(_STIXBase):
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709267>`__.
     """
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('kill_chain_name', StringProperty(required=True)),
         ('phase_name', StringProperty(required=True)),
     ])
@@ -49,9 +47,8 @@ class GranularMarking(_STIXBase):
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part1-stix-core/stix-v2.0-cs01-part1-stix-core.html#_Toc496709290>`__.
     """
 
-    _properties = OrderedDict()
-    _properties.update([
-        ('marking_ref', ReferenceProperty(required=True, type="marking-definition")),
+    _properties = OrderedDict([
+        ('marking_ref', ReferenceProperty(required=True, type='marking-definition')),
         ('selectors', ListProperty(SelectorProperty, required=True)),
     ])
 
@@ -64,8 +61,7 @@ class TLPMarking(_STIXBase):
 
     # TODO: don't allow the creation of any other TLPMarkings than the ones below
     _type = 'tlp'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('tlp', StringProperty(required=True))
     ])
 
@@ -77,8 +73,7 @@ class StatementMarking(_STIXBase):
     """
 
     _type = 'statement'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('statement', StringProperty(required=True))
     ])
 
@@ -109,14 +104,13 @@ class MarkingDefinition(_STIXBase, _MarkingsMixin):
     """
 
     _type = 'marking-definition'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
         ('definition_type', StringProperty(required=True)),
         ('definition', MarkingProperty(required=True)),
@@ -193,29 +187,29 @@ def CustomMarking(type='x-custom-marking', properties=None):
 # TODO: don't allow the creation of any other TLPMarkings than the ones below
 
 TLP_WHITE = MarkingDefinition(
-    id="marking-definition--613f2e26-407d-48c7-9eca-b8e91df99dc9",
-    created="2017-01-20T00:00:00.000Z",
-    definition_type="tlp",
-    definition=TLPMarking(tlp="white")
+    id='marking-definition--613f2e26-407d-48c7-9eca-b8e91df99dc9',
+    created='2017-01-20T00:00:00.000Z',
+    definition_type='tlp',
+    definition=TLPMarking(tlp='white')
 )
 
 TLP_GREEN = MarkingDefinition(
-    id="marking-definition--34098fce-860f-48ae-8e50-ebd3cc5e41da",
-    created="2017-01-20T00:00:00.000Z",
-    definition_type="tlp",
-    definition=TLPMarking(tlp="green")
+    id='marking-definition--34098fce-860f-48ae-8e50-ebd3cc5e41da',
+    created='2017-01-20T00:00:00.000Z',
+    definition_type='tlp',
+    definition=TLPMarking(tlp='green')
 )
 
 TLP_AMBER = MarkingDefinition(
-    id="marking-definition--f88d31f6-486f-44da-b317-01333bde0b82",
-    created="2017-01-20T00:00:00.000Z",
-    definition_type="tlp",
-    definition=TLPMarking(tlp="amber")
+    id='marking-definition--f88d31f6-486f-44da-b317-01333bde0b82',
+    created='2017-01-20T00:00:00.000Z',
+    definition_type='tlp',
+    definition=TLPMarking(tlp='amber')
 )
 
 TLP_RED = MarkingDefinition(
-    id="marking-definition--5e57c739-391a-4eb3-b6be-7d15ca92d5ed",
-    created="2017-01-20T00:00:00.000Z",
-    definition_type="tlp",
-    definition=TLPMarking(tlp="red")
+    id='marking-definition--5e57c739-391a-4eb3-b6be-7d15ca92d5ed',
+    created='2017-01-20T00:00:00.000Z',
+    definition_type='tlp',
+    definition=TLPMarking(tlp='red')
 )
diff --git a/stix2/v20/observables.py b/stix2/v20/observables.py
index 1cfebad..6eec82c 100644
--- a/stix2/v20/observables.py
+++ b/stix2/v20/observables.py
@@ -102,8 +102,7 @@ class Artifact(_Observable):
     """  # noqa
 
     _type = 'artifact'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('mime_type', StringProperty()),
         ('payload_bin', BinaryProperty()),
@@ -114,8 +113,8 @@ class Artifact(_Observable):
 
     def _check_object_constraints(self):
         super(Artifact, self)._check_object_constraints()
-        self._check_mutually_exclusive_properties(["payload_bin", "url"])
-        self._check_properties_dependency(["hashes"], ["url"])
+        self._check_mutually_exclusive_properties(['payload_bin', 'url'])
+        self._check_properties_dependency(['hashes'], ['url'])
 
 
 class AutonomousSystem(_Observable):
@@ -124,8 +123,7 @@ class AutonomousSystem(_Observable):
     """  # noqa
 
     _type = 'autonomous-system'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('number', IntegerProperty(required=True)),
         ('name', StringProperty()),
@@ -140,8 +138,7 @@ class Directory(_Observable):
     """  # noqa
 
     _type = 'directory'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('path', StringProperty(required=True)),
         ('path_enc', StringProperty()),
@@ -160,8 +157,7 @@ class DomainName(_Observable):
     """  # noqa
 
     _type = 'domain-name'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('resolves_to_refs', ListProperty(ObjectReferenceProperty(valid_types=['ipv4-addr', 'ipv6-addr', 'domain-name']))),
@@ -175,8 +171,7 @@ class EmailAddress(_Observable):
     """  # noqa
 
     _type = 'email-addr'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('display_name', StringProperty()),
@@ -190,8 +185,7 @@ class EmailMIMEComponent(_STIXBase):
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part4-cyber-observable-objects/stix-v2.0-cs01-part4-cyber-observable-objects.html#_Toc496716231>`__.
     """  # noqa
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('body', StringProperty()),
         ('body_raw_ref', ObjectReferenceProperty(valid_types=['artifact', 'file'])),
         ('content_type', StringProperty()),
@@ -200,7 +194,7 @@ class EmailMIMEComponent(_STIXBase):
 
     def _check_object_constraints(self):
         super(EmailMIMEComponent, self)._check_object_constraints()
-        self._check_at_least_one_property(["body", "body_raw_ref"])
+        self._check_at_least_one_property(['body', 'body_raw_ref'])
 
 
 class EmailMessage(_Observable):
@@ -209,8 +203,7 @@ class EmailMessage(_Observable):
     """  # noqa
 
     _type = 'email-message'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('is_multipart', BooleanProperty(required=True)),
         ('date', TimestampProperty()),
@@ -231,10 +224,10 @@ class EmailMessage(_Observable):
 
     def _check_object_constraints(self):
         super(EmailMessage, self)._check_object_constraints()
-        self._check_properties_dependency(["is_multipart"], ["body_multipart"])
-        if self.get("is_multipart") is True and self.get("body"):
+        self._check_properties_dependency(['is_multipart'], ['body_multipart'])
+        if self.get('is_multipart') is True and self.get('body'):
             # 'body' MAY only be used if is_multipart is false.
-            raise DependentPropertiesError(self.__class__, [("is_multipart", "body")])
+            raise DependentPropertiesError(self.__class__, [('is_multipart', 'body')])
 
 
 class ArchiveExt(_Extension):
@@ -243,8 +236,7 @@ class ArchiveExt(_Extension):
     """  # noqa
 
     _type = 'archive-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('contains_refs', ListProperty(ObjectReferenceProperty(valid_types='file'), required=True)),
         ('version', StringProperty()),
         ('comment', StringProperty()),
@@ -256,8 +248,7 @@ class AlternateDataStream(_STIXBase):
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part4-cyber-observable-objects/stix-v2.0-cs01-part4-cyber-observable-objects.html#_Toc496716239>`__.
     """  # noqa
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('name', StringProperty(required=True)),
         ('hashes', HashesProperty()),
         ('size', IntegerProperty()),
@@ -270,8 +261,7 @@ class NTFSExt(_Extension):
     """  # noqa
 
     _type = 'ntfs-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('sid', StringProperty()),
         ('alternate_data_streams', ListProperty(EmbeddedObjectProperty(type=AlternateDataStream))),
     ])
@@ -283,8 +273,7 @@ class PDFExt(_Extension):
     """  # noqa
 
     _type = 'pdf-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('version', StringProperty()),
         ('is_optimized', BooleanProperty()),
         ('document_info_dict', DictionaryProperty()),
@@ -299,8 +288,7 @@ class RasterImageExt(_Extension):
     """  # noqa
 
     _type = 'raster-image-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('image_height', IntegerProperty()),
         ('image_width', IntegerProperty()),
         ('bits_per_pixel', IntegerProperty()),
@@ -314,8 +302,7 @@ class WindowsPEOptionalHeaderType(_STIXBase):
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part4-cyber-observable-objects/stix-v2.0-cs01-part4-cyber-observable-objects.html#_Toc496716248>`__.
     """  # noqa
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('magic_hex', HexProperty()),
         ('major_linker_version', IntegerProperty()),
         ('minor_linker_version', IntegerProperty()),
@@ -359,8 +346,7 @@ class WindowsPESection(_STIXBase):
     `the STIX 2.0 specification <http://docs.oasis-open.org/cti/stix/v2.0/cs01/part4-cyber-observable-objects/stix-v2.0-cs01-part4-cyber-observable-objects.html#_Toc496716250>`__.
     """  # noqa
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('name', StringProperty(required=True)),
         ('size', IntegerProperty()),
         ('entropy', FloatProperty()),
@@ -374,8 +360,7 @@ class WindowsPEBinaryExt(_Extension):
     """  # noqa
 
     _type = 'windows-pebinary-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('pe_type', StringProperty(required=True)),  # open_vocab
         ('imphash', StringProperty()),
         ('machine_hex', HexProperty()),
@@ -397,8 +382,7 @@ class File(_Observable):
     """  # noqa
 
     _type = 'file'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('hashes', HashesProperty()),
         ('size', IntegerProperty()),
@@ -421,8 +405,8 @@ class File(_Observable):
 
     def _check_object_constraints(self):
         super(File, self)._check_object_constraints()
-        self._check_properties_dependency(["is_encrypted"], ["encryption_algorithm", "decryption_key"])
-        self._check_at_least_one_property(["hashes", "name"])
+        self._check_properties_dependency(['is_encrypted'], ['encryption_algorithm', 'decryption_key'])
+        self._check_at_least_one_property(['hashes', 'name'])
 
 
 class IPv4Address(_Observable):
@@ -431,8 +415,7 @@ class IPv4Address(_Observable):
     """  # noqa
 
     _type = 'ipv4-addr'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('resolves_to_refs', ListProperty(ObjectReferenceProperty(valid_types='mac-addr'))),
@@ -447,8 +430,7 @@ class IPv6Address(_Observable):
     """  # noqa
 
     _type = 'ipv6-addr'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('resolves_to_refs', ListProperty(ObjectReferenceProperty(valid_types='mac-addr'))),
@@ -463,8 +445,7 @@ class MACAddress(_Observable):
     """  # noqa
 
     _type = 'mac-addr'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('extensions', ExtensionsProperty(enclosing_type=_type)),
@@ -477,8 +458,7 @@ class Mutex(_Observable):
     """  # noqa
 
     _type = 'mutex'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('name', StringProperty(required=True)),
         ('extensions', ExtensionsProperty(enclosing_type=_type)),
@@ -491,8 +471,7 @@ class HTTPRequestExt(_Extension):
     """  # noqa
 
     _type = 'http-request-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('request_method', StringProperty(required=True)),
         ('request_value', StringProperty(required=True)),
         ('request_version', StringProperty()),
@@ -508,8 +487,7 @@ class ICMPExt(_Extension):
     """  # noqa
 
     _type = 'icmp-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('icmp_type_hex', HexProperty(required=True)),
         ('icmp_code_hex', HexProperty(required=True)),
     ])
@@ -521,8 +499,7 @@ class SocketExt(_Extension):
     """  # noqa
 
     _type = 'socket-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('address_family', EnumProperty(allowed=[
             "AF_UNSPEC",
             "AF_INET",
@@ -562,8 +539,7 @@ class TCPExt(_Extension):
     """  # noqa
 
     _type = 'tcp-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('src_flags_hex', HexProperty()),
         ('dst_flags_hex', HexProperty()),
     ])
@@ -575,8 +551,7 @@ class NetworkTraffic(_Observable):
     """  # noqa
 
     _type = 'network-traffic'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('start', TimestampProperty()),
         ('end', TimestampProperty()),
@@ -600,7 +575,7 @@ class NetworkTraffic(_Observable):
 
     def _check_object_constraints(self):
         super(NetworkTraffic, self)._check_object_constraints()
-        self._check_at_least_one_property(["src_ref", "dst_ref"])
+        self._check_at_least_one_property(['src_ref', 'dst_ref'])
 
 
 class WindowsProcessExt(_Extension):
@@ -609,8 +584,7 @@ class WindowsProcessExt(_Extension):
     """  # noqa
 
     _type = 'windows-process-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('aslr_enabled', BooleanProperty()),
         ('dep_enabled', BooleanProperty()),
         ('priority', StringProperty()),
@@ -626,8 +600,7 @@ class WindowsServiceExt(_Extension):
     """  # noqa
 
     _type = 'windows-service-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('service_name', StringProperty(required=True)),
         ('descriptions', ListProperty(StringProperty)),
         ('display_name', StringProperty()),
@@ -664,8 +637,7 @@ class Process(_Observable):
     """  # noqa
 
     _type = 'process'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('is_hidden', BooleanProperty()),
         ('pid', IntegerProperty()),
@@ -689,14 +661,14 @@ class Process(_Observable):
         super(Process, self)._check_object_constraints()
         try:
             self._check_at_least_one_property()
-            if "windows-process-ext" in self.get('extensions', {}):
-                self.extensions["windows-process-ext"]._check_at_least_one_property()
+            if 'windows-process-ext' in self.get('extensions', {}):
+                self.extensions['windows-process-ext']._check_at_least_one_property()
         except AtLeastOnePropertyError as enclosing_exc:
             if 'extensions' not in self:
                 raise enclosing_exc
             else:
-                if "windows-process-ext" in self.get('extensions', {}):
-                    self.extensions["windows-process-ext"]._check_at_least_one_property()
+                if 'windows-process-ext' in self.get('extensions', {}):
+                    self.extensions['windows-process-ext']._check_at_least_one_property()
 
 
 class Software(_Observable):
@@ -705,8 +677,7 @@ class Software(_Observable):
     """  # noqa
 
     _type = 'software'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('name', StringProperty(required=True)),
         ('cpe', StringProperty()),
@@ -723,8 +694,7 @@ class URL(_Observable):
     """  # noqa
 
     _type = 'url'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('extensions', ExtensionsProperty(enclosing_type=_type)),
@@ -737,8 +707,7 @@ class UNIXAccountExt(_Extension):
     """  # noqa
 
     _type = 'unix-account-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('gid', IntegerProperty()),
         ('groups', ListProperty(StringProperty)),
         ('home_dir', StringProperty()),
@@ -752,8 +721,7 @@ class UserAccount(_Observable):
     """  # noqa
 
     _type = 'user-account'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('user_id', StringProperty(required=True)),
         ('account_login', StringProperty()),
@@ -778,24 +746,23 @@ class WindowsRegistryValueType(_STIXBase):
     """  # noqa
 
     _type = 'windows-registry-value-type'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('name', StringProperty(required=True)),
         ('data', StringProperty()),
         ('data_type', EnumProperty(allowed=[
-            'REG_NONE',
-            'REG_SZ',
-            'REG_EXPAND_SZ',
-            'REG_BINARY',
-            'REG_DWORD',
-            'REG_DWORD_BIG_ENDIAN',
-            'REG_LINK',
-            'REG_MULTI_SZ',
-            'REG_RESOURCE_LIST',
-            'REG_FULL_RESOURCE_DESCRIPTION',
-            'REG_RESOURCE_REQUIREMENTS_LIST',
-            'REG_QWORD',
-            'REG_INVALID_TYPE',
+            "REG_NONE",
+            "REG_SZ",
+            "REG_EXPAND_SZ",
+            "REG_BINARY",
+            "REG_DWORD",
+            "REG_DWORD_BIG_ENDIAN",
+            "REG_LINK",
+            "REG_MULTI_SZ",
+            "REG_RESOURCE_LIST",
+            "REG_FULL_RESOURCE_DESCRIPTION",
+            "REG_RESOURCE_REQUIREMENTS_LIST",
+            "REG_QWORD",
+            "REG_INVALID_TYPE",
         ])),
     ])
 
@@ -806,8 +773,7 @@ class WindowsRegistryKey(_Observable):
     """  # noqa
 
     _type = 'windows-registry-key'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('key', StringProperty(required=True)),
         ('values', ListProperty(EmbeddedObjectProperty(type=WindowsRegistryValueType))),
@@ -830,8 +796,7 @@ class X509V3ExtenstionsType(_STIXBase):
     """  # noqa
 
     _type = 'x509-v3-extensions-type'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('basic_constraints', StringProperty()),
         ('name_constraints', StringProperty()),
         ('policy_constraints', StringProperty()),
@@ -857,8 +822,7 @@ class X509Certificate(_Observable):
     """  # noqa
 
     _type = 'x509-certificate'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('is_self_signed', BooleanProperty()),
         ('hashes', HashesProperty()),
diff --git a/stix2/v20/sdo.py b/stix2/v20/sdo.py
index 5eb0801..142cb4f 100644
--- a/stix2/v20/sdo.py
+++ b/stix2/v20/sdo.py
@@ -25,11 +25,10 @@ class AttackPattern(STIXDomainObject):
     """
 
     _type = 'attack-pattern'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -38,7 +37,7 @@ class AttackPattern(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -49,11 +48,10 @@ class Campaign(STIXDomainObject):
     """
 
     _type = 'campaign'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -65,7 +63,7 @@ class Campaign(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -76,11 +74,10 @@ class CourseOfAction(STIXDomainObject):
     """
 
     _type = 'course-of-action'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -88,7 +85,7 @@ class CourseOfAction(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -99,11 +96,10 @@ class Identity(STIXDomainObject):
     """
 
     _type = 'identity'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -114,7 +110,7 @@ class Identity(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -125,11 +121,10 @@ class Indicator(STIXDomainObject):
     """
 
     _type = 'indicator'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty()),
@@ -141,7 +136,7 @@ class Indicator(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty, required=True)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -152,11 +147,10 @@ class IntrusionSet(STIXDomainObject):
     """
 
     _type = 'intrusion-set'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -171,7 +165,7 @@ class IntrusionSet(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -182,11 +176,10 @@ class Malware(STIXDomainObject):
     """
 
     _type = 'malware'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -195,7 +188,7 @@ class Malware(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty, required=True)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -206,11 +199,10 @@ class ObservedData(STIXDomainObject):
     """
 
     _type = 'observed-data'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('first_observed', TimestampProperty(required=True)),
@@ -220,7 +212,7 @@ class ObservedData(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -237,11 +229,10 @@ class Report(STIXDomainObject):
     """
 
     _type = 'report'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -251,7 +242,7 @@ class Report(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty, required=True)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -262,11 +253,10 @@ class ThreatActor(STIXDomainObject):
     """
 
     _type = 'threat-actor'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -282,7 +272,7 @@ class ThreatActor(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty, required=True)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -293,11 +283,10 @@ class Tool(STIXDomainObject):
     """
 
     _type = 'tool'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -307,7 +296,7 @@ class Tool(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty, required=True)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -318,11 +307,10 @@ class Vulnerability(STIXDomainObject):
     """
 
     _type = 'vulnerability'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -330,7 +318,7 @@ class Vulnerability(STIXDomainObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -371,11 +359,10 @@ def CustomObject(type='x-custom-type', properties=None):
                 raise ValueError("Invalid type name '%s': must be between 3 and 250 characters." % type)
 
             _type = type
-            _properties = OrderedDict()
-            _properties.update([
+            _properties = OrderedDict([
                 ('type', TypeProperty(_type)),
                 ('id', IDProperty(_type)),
-                ('created_by_ref', ReferenceProperty(type="identity")),
+                ('created_by_ref', ReferenceProperty(type='identity')),
                 ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
                 ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
             ])
@@ -383,19 +370,19 @@ def CustomObject(type='x-custom-type', properties=None):
             if not properties or not isinstance(properties, list):
                 raise ValueError("Must supply a list, containing tuples. For example, [('property1', IntegerProperty())]")
 
-            _properties.update([x for x in properties if not x[0].startswith("x_")])
+            _properties.update([x for x in properties if not x[0].startswith('x_')])
 
             # This is to follow the general properties structure.
             _properties.update([
                 ('revoked', BooleanProperty(default=lambda: False)),
                 ('labels', ListProperty(StringProperty)),
                 ('external_references', ListProperty(ExternalReference)),
-                ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+                ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
                 ('granular_markings', ListProperty(GranularMarking)),
             ])
 
             # Put all custom properties at the bottom, sorted alphabetically.
-            _properties.update(sorted([x for x in properties if x[0].startswith("x_")], key=lambda x: x[0]))
+            _properties.update(sorted([x for x in properties if x[0].startswith('x_')], key=lambda x: x[0]))
 
             def __init__(self, **kwargs):
                 _STIXBase.__init__(self, **kwargs)
@@ -408,7 +395,7 @@ def CustomObject(type='x-custom-type', properties=None):
                         return
                     raise e
 
-        _register_type(_Custom, version="2.0")
+        _register_type(_Custom, version='2.0')
         return _Custom
 
     return custom_builder
diff --git a/stix2/v20/sro.py b/stix2/v20/sro.py
index 89520ac..5557127 100644
--- a/stix2/v20/sro.py
+++ b/stix2/v20/sro.py
@@ -21,11 +21,10 @@ class Relationship(STIXRelationshipObject):
     """
 
     _type = 'relationship'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('relationship_type', StringProperty(required=True)),
@@ -35,7 +34,7 @@ class Relationship(STIXRelationshipObject):
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -59,24 +58,23 @@ class Sighting(STIXRelationshipObject):
     """
 
     _type = 'sighting'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('first_seen', TimestampProperty()),
         ('last_seen', TimestampProperty()),
         ('count', IntegerProperty()),
         ('sighting_of_ref', ReferenceProperty(required=True)),
-        ('observed_data_refs', ListProperty(ReferenceProperty(type="observed-data"))),
-        ('where_sighted_refs', ListProperty(ReferenceProperty(type="identity"))),
+        ('observed_data_refs', ListProperty(ReferenceProperty(type='observed-data'))),
+        ('where_sighted_refs', ListProperty(ReferenceProperty(type='identity'))),
         ('summary', BooleanProperty(default=lambda: False)),
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
diff --git a/stix2/v21/bundle.py b/stix2/v21/bundle.py
index 1957bf1..794b8a0 100644
--- a/stix2/v21/bundle.py
+++ b/stix2/v21/bundle.py
@@ -39,8 +39,7 @@ class Bundle(_STIXBase):
     """
 
     _type = 'bundle'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
         ('objects', ListProperty(STIXObjectProperty)),
diff --git a/stix2/v21/common.py b/stix2/v21/common.py
index 2bde828..a47418e 100644
--- a/stix2/v21/common.py
+++ b/stix2/v21/common.py
@@ -13,8 +13,7 @@ from .properties import (BooleanProperty, DictionaryProperty, HashesProperty,
 
 class ExternalReference(_STIXBase):
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('source_name', StringProperty(required=True)),
         ('description', StringProperty()),
         ('url', StringProperty()),
@@ -24,13 +23,12 @@ class ExternalReference(_STIXBase):
 
     def _check_object_constraints(self):
         super(ExternalReference, self)._check_object_constraints()
-        self._check_at_least_one_property(["description", "external_id", "url"])
+        self._check_at_least_one_property(['description', 'external_id', 'url'])
 
 
 class KillChainPhase(_STIXBase):
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('kill_chain_name', StringProperty(required=True)),
         ('phase_name', StringProperty(required=True)),
     ])
@@ -38,26 +36,24 @@ class KillChainPhase(_STIXBase):
 
 class GranularMarking(_STIXBase):
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('lang', StringProperty()),
-        ('marking_ref', ReferenceProperty(type="marking-definition")),
+        ('marking_ref', ReferenceProperty(type='marking-definition')),
         ('selectors', ListProperty(SelectorProperty, required=True)),
     ])
 
     def _check_object_constraints(self):
         super(GranularMarking, self)._check_object_constraints()
-        self._check_at_least_one_property(["lang", "marking_ref"])
+        self._check_at_least_one_property(['lang', 'marking_ref'])
 
 
 class LanguageContent(_STIXBase):
 
     _type = 'language-content'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('object_ref', ReferenceProperty(required=True)),
@@ -68,7 +64,7 @@ class LanguageContent(_STIXBase):
         ('revoked', BooleanProperty()),
         ('labels', ListProperty(StringProperty)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -76,8 +72,7 @@ class LanguageContent(_STIXBase):
 class TLPMarking(_STIXBase):
 
     _type = 'tlp'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('tlp', Property(required=True))
     ])
 
@@ -85,8 +80,7 @@ class TLPMarking(_STIXBase):
 class StatementMarking(_STIXBase):
 
     _type = 'statement'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('statement', StringProperty(required=True))
     ])
 
@@ -113,14 +107,13 @@ class MarkingProperty(Property):
 class MarkingDefinition(_STIXBase, _MarkingsMixin):
 
     _type = 'marking-definition'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW)),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
         ('definition_type', StringProperty(required=True)),
         ('definition', MarkingProperty(required=True)),
@@ -170,12 +163,11 @@ def CustomMarking(type='x-custom-marking', properties=None):
 
         class _Custom(cls, _STIXBase):
             _type = type
-            _properties = OrderedDict()
 
             if not properties or not isinstance(properties, list):
                 raise ValueError("Must supply a list, containing tuples. For example, [('property1', IntegerProperty())]")
 
-            _properties.update(properties)
+            _properties = OrderedDict(properties)
 
             def __init__(self, **kwargs):
                 _STIXBase.__init__(self, **kwargs)
@@ -197,29 +189,29 @@ def CustomMarking(type='x-custom-marking', properties=None):
 # TODO: don't allow the creation of any other TLPMarkings than the ones below
 
 TLP_WHITE = MarkingDefinition(
-    id="marking-definition--613f2e26-407d-48c7-9eca-b8e91df99dc9",
-    created="2017-01-20T00:00:00.000Z",
-    definition_type="tlp",
-    definition=TLPMarking(tlp="white")
+    id='marking-definition--613f2e26-407d-48c7-9eca-b8e91df99dc9',
+    created='2017-01-20T00:00:00.000Z',
+    definition_type='tlp',
+    definition=TLPMarking(tlp='white')
 )
 
 TLP_GREEN = MarkingDefinition(
-    id="marking-definition--34098fce-860f-48ae-8e50-ebd3cc5e41da",
-    created="2017-01-20T00:00:00.000Z",
-    definition_type="tlp",
-    definition=TLPMarking(tlp="green")
+    id='marking-definition--34098fce-860f-48ae-8e50-ebd3cc5e41da',
+    created='2017-01-20T00:00:00.000Z',
+    definition_type='tlp',
+    definition=TLPMarking(tlp='green')
 )
 
 TLP_AMBER = MarkingDefinition(
-    id="marking-definition--f88d31f6-486f-44da-b317-01333bde0b82",
-    created="2017-01-20T00:00:00.000Z",
-    definition_type="tlp",
-    definition=TLPMarking(tlp="amber")
+    id='marking-definition--f88d31f6-486f-44da-b317-01333bde0b82',
+    created='2017-01-20T00:00:00.000Z',
+    definition_type='tlp',
+    definition=TLPMarking(tlp='amber')
 )
 
 TLP_RED = MarkingDefinition(
-    id="marking-definition--5e57c739-391a-4eb3-b6be-7d15ca92d5ed",
-    created="2017-01-20T00:00:00.000Z",
-    definition_type="tlp",
-    definition=TLPMarking(tlp="red")
+    id='marking-definition--5e57c739-391a-4eb3-b6be-7d15ca92d5ed',
+    created='2017-01-20T00:00:00.000Z',
+    definition_type='tlp',
+    definition=TLPMarking(tlp='red')
 )
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index 052d0d9..f4fb561 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -103,8 +103,7 @@ class Artifact(_Observable):
     """
 
     _type = 'artifact'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('mime_type', StringProperty()),
         ('payload_bin', BinaryProperty()),
@@ -115,8 +114,8 @@ class Artifact(_Observable):
 
     def _check_object_constraints(self):
         super(Artifact, self)._check_object_constraints()
-        self._check_mutually_exclusive_properties(["payload_bin", "url"])
-        self._check_properties_dependency(["hashes"], ["url"])
+        self._check_mutually_exclusive_properties(['payload_bin', 'url'])
+        self._check_properties_dependency(['hashes'], ['url'])
 
 
 class AutonomousSystem(_Observable):
@@ -126,8 +125,7 @@ class AutonomousSystem(_Observable):
     """
 
     _type = 'autonomous-system'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('number', IntegerProperty(required=True)),
         ('name', StringProperty()),
@@ -143,8 +141,7 @@ class Directory(_Observable):
     """
 
     _type = 'directory'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('path', StringProperty(required=True)),
         ('path_enc', StringProperty()),
@@ -164,8 +161,7 @@ class DomainName(_Observable):
     """
 
     _type = 'domain-name'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('resolves_to_refs', ListProperty(ObjectReferenceProperty(valid_types=['ipv4-addr', 'ipv6-addr', 'domain-name']))),
@@ -180,8 +176,7 @@ class EmailAddress(_Observable):
     """
 
     _type = 'email-addr'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('display_name', StringProperty()),
@@ -196,8 +191,7 @@ class EmailMIMEComponent(_STIXBase):
     `the STIX 2.1 specification <link here>`__.
     """
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('body', StringProperty()),
         ('body_raw_ref', ObjectReferenceProperty(valid_types=['artifact', 'file'])),
         ('content_type', StringProperty()),
@@ -206,7 +200,7 @@ class EmailMIMEComponent(_STIXBase):
 
     def _check_object_constraints(self):
         super(EmailMIMEComponent, self)._check_object_constraints()
-        self._check_at_least_one_property(["body", "body_raw_ref"])
+        self._check_at_least_one_property(['body', 'body_raw_ref'])
 
 
 class EmailMessage(_Observable):
@@ -216,8 +210,7 @@ class EmailMessage(_Observable):
     """
 
     _type = 'email-message'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('is_multipart', BooleanProperty(required=True)),
         ('date', TimestampProperty()),
@@ -238,10 +231,10 @@ class EmailMessage(_Observable):
 
     def _check_object_constraints(self):
         super(EmailMessage, self)._check_object_constraints()
-        self._check_properties_dependency(["is_multipart"], ["body_multipart"])
-        if self.get("is_multipart") is True and self.get("body"):
+        self._check_properties_dependency(['is_multipart'], ['body_multipart'])
+        if self.get('is_multipart') is True and self.get('body'):
             # 'body' MAY only be used if is_multipart is false.
-            raise DependentPropertiesError(self.__class__, [("is_multipart", "body")])
+            raise DependentPropertiesError(self.__class__, [('is_multipart', 'body')])
 
 
 class ArchiveExt(_Extension):
@@ -251,8 +244,7 @@ class ArchiveExt(_Extension):
     """
 
     _type = 'archive-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('contains_refs', ListProperty(ObjectReferenceProperty(valid_types='file'), required=True)),
         ('version', StringProperty()),
         ('comment', StringProperty()),
@@ -265,8 +257,7 @@ class AlternateDataStream(_STIXBase):
     `the STIX 2.1 specification <link here>`__.
     """
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('name', StringProperty(required=True)),
         ('hashes', HashesProperty()),
         ('size', IntegerProperty()),
@@ -280,8 +271,7 @@ class NTFSExt(_Extension):
     """
 
     _type = 'ntfs-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('sid', StringProperty()),
         ('alternate_data_streams', ListProperty(EmbeddedObjectProperty(type=AlternateDataStream))),
     ])
@@ -294,8 +284,7 @@ class PDFExt(_Extension):
     """
 
     _type = 'pdf-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('version', StringProperty()),
         ('is_optimized', BooleanProperty()),
         ('document_info_dict', DictionaryProperty()),
@@ -311,8 +300,7 @@ class RasterImageExt(_Extension):
     """
 
     _type = 'raster-image-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('image_height', IntegerProperty()),
         ('image_width', IntegerProperty()),
         ('bits_per_pixel', IntegerProperty()),
@@ -327,8 +315,7 @@ class WindowsPEOptionalHeaderType(_STIXBase):
     `the STIX 2.1 specification <link here>`__.
     """
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('magic_hex', HexProperty()),
         ('major_linker_version', IntegerProperty()),
         ('minor_linker_version', IntegerProperty()),
@@ -373,8 +360,7 @@ class WindowsPESection(_STIXBase):
     `the STIX 2.1 specification <link here>`__.
     """
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('name', StringProperty(required=True)),
         ('size', IntegerProperty()),
         ('entropy', FloatProperty()),
@@ -389,8 +375,7 @@ class WindowsPEBinaryExt(_Extension):
     """
 
     _type = 'windows-pebinary-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('pe_type', StringProperty(required=True)),  # open_vocab
         ('imphash', StringProperty()),
         ('machine_hex', HexProperty()),
@@ -413,8 +398,7 @@ class File(_Observable):
     """
 
     _type = 'file'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('hashes', HashesProperty()),
         ('size', IntegerProperty()),
@@ -437,8 +421,8 @@ class File(_Observable):
 
     def _check_object_constraints(self):
         super(File, self)._check_object_constraints()
-        self._check_properties_dependency(["is_encrypted"], ["encryption_algorithm", "decryption_key"])
-        self._check_at_least_one_property(["hashes", "name"])
+        self._check_properties_dependency(['is_encrypted'], ['encryption_algorithm', 'decryption_key'])
+        self._check_at_least_one_property(['hashes', 'name'])
 
 
 class IPv4Address(_Observable):
@@ -448,8 +432,7 @@ class IPv4Address(_Observable):
     """
 
     _type = 'ipv4-addr'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('resolves_to_refs', ListProperty(ObjectReferenceProperty(valid_types='mac-addr'))),
@@ -465,8 +448,7 @@ class IPv6Address(_Observable):
     """
 
     _type = 'ipv6-addr'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('resolves_to_refs', ListProperty(ObjectReferenceProperty(valid_types='mac-addr'))),
@@ -482,8 +464,7 @@ class MACAddress(_Observable):
     """
 
     _type = 'mac-addr'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('extensions', ExtensionsProperty(enclosing_type=_type)),
@@ -497,8 +478,7 @@ class Mutex(_Observable):
     """
 
     _type = 'mutex'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('name', StringProperty(required=True)),
         ('extensions', ExtensionsProperty(enclosing_type=_type)),
@@ -512,8 +492,7 @@ class HTTPRequestExt(_Extension):
     """
 
     _type = 'http-request-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('request_method', StringProperty(required=True)),
         ('request_value', StringProperty(required=True)),
         ('request_version', StringProperty()),
@@ -530,8 +509,7 @@ class ICMPExt(_Extension):
     """
 
     _type = 'icmp-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('icmp_type_hex', HexProperty(required=True)),
         ('icmp_code_hex', HexProperty(required=True)),
     ])
@@ -544,8 +522,7 @@ class SocketExt(_Extension):
     """
 
     _type = 'socket-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('address_family', EnumProperty(allowed=[
             "AF_UNSPEC",
             "AF_INET",
@@ -586,8 +563,7 @@ class TCPExt(_Extension):
     """
 
     _type = 'tcp-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('src_flags_hex', HexProperty()),
         ('dst_flags_hex', HexProperty()),
     ])
@@ -600,8 +576,7 @@ class NetworkTraffic(_Observable):
     """
 
     _type = 'network-traffic'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('start', TimestampProperty()),
         ('end', TimestampProperty()),
@@ -625,7 +600,7 @@ class NetworkTraffic(_Observable):
 
     def _check_object_constraints(self):
         super(NetworkTraffic, self)._check_object_constraints()
-        self._check_at_least_one_property(["src_ref", "dst_ref"])
+        self._check_at_least_one_property(['src_ref', 'dst_ref'])
 
 
 class WindowsProcessExt(_Extension):
@@ -635,8 +610,7 @@ class WindowsProcessExt(_Extension):
     """
 
     _type = 'windows-process-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('aslr_enabled', BooleanProperty()),
         ('dep_enabled', BooleanProperty()),
         ('priority', StringProperty()),
@@ -653,8 +627,7 @@ class WindowsServiceExt(_Extension):
     """
 
     _type = 'windows-service-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('service_name', StringProperty(required=True)),
         ('descriptions', ListProperty(StringProperty)),
         ('display_name', StringProperty()),
@@ -692,8 +665,7 @@ class Process(_Observable):
     """
 
     _type = 'process'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('is_hidden', BooleanProperty()),
         ('pid', IntegerProperty()),
@@ -717,14 +689,14 @@ class Process(_Observable):
         super(Process, self)._check_object_constraints()
         try:
             self._check_at_least_one_property()
-            if "windows-process-ext" in self.get('extensions', {}):
-                self.extensions["windows-process-ext"]._check_at_least_one_property()
+            if 'windows-process-ext' in self.get('extensions', {}):
+                self.extensions['windows-process-ext']._check_at_least_one_property()
         except AtLeastOnePropertyError as enclosing_exc:
             if 'extensions' not in self:
                 raise enclosing_exc
             else:
-                if "windows-process-ext" in self.get('extensions', {}):
-                    self.extensions["windows-process-ext"]._check_at_least_one_property()
+                if 'windows-process-ext' in self.get('extensions', {}):
+                    self.extensions['windows-process-ext']._check_at_least_one_property()
 
 
 class Software(_Observable):
@@ -734,8 +706,7 @@ class Software(_Observable):
     """
 
     _type = 'software'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('name', StringProperty(required=True)),
         ('cpe', StringProperty()),
@@ -753,8 +724,7 @@ class URL(_Observable):
     """
 
     _type = 'url'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('value', StringProperty(required=True)),
         ('extensions', ExtensionsProperty(enclosing_type=_type)),
@@ -768,8 +738,7 @@ class UNIXAccountExt(_Extension):
     """
 
     _type = 'unix-account-ext'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('gid', IntegerProperty()),
         ('groups', ListProperty(StringProperty)),
         ('home_dir', StringProperty()),
@@ -784,8 +753,7 @@ class UserAccount(_Observable):
     """
 
     _type = 'user-account'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('user_id', StringProperty(required=True)),
         ('account_login', StringProperty()),
@@ -811,24 +779,23 @@ class WindowsRegistryValueType(_STIXBase):
     """
 
     _type = 'windows-registry-value-type'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('name', StringProperty(required=True)),
         ('data', StringProperty()),
         ('data_type', EnumProperty(allowed=[
-            'REG_NONE',
-            'REG_SZ',
-            'REG_EXPAND_SZ',
-            'REG_BINARY',
-            'REG_DWORD',
-            'REG_DWORD_BIG_ENDIAN',
-            'REG_LINK',
-            'REG_MULTI_SZ',
-            'REG_RESOURCE_LIST',
-            'REG_FULL_RESOURCE_DESCRIPTION',
-            'REG_RESOURCE_REQUIREMENTS_LIST',
-            'REG_QWORD',
-            'REG_INVALID_TYPE',
+            "REG_NONE",
+            "REG_SZ",
+            "REG_EXPAND_SZ",
+            "REG_BINARY",
+            "REG_DWORD",
+            "REG_DWORD_BIG_ENDIAN",
+            "REG_LINK",
+            "REG_MULTI_SZ",
+            "REG_RESOURCE_LIST",
+            "REG_FULL_RESOURCE_DESCRIPTION",
+            "REG_RESOURCE_REQUIREMENTS_LIST",
+            "REG_QWORD",
+            "REG_INVALID_TYPE",
         ])),
     ])
 
@@ -840,8 +807,7 @@ class WindowsRegistryKey(_Observable):
     """
 
     _type = 'windows-registry-key'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('key', StringProperty(required=True)),
         ('values', ListProperty(EmbeddedObjectProperty(type=WindowsRegistryValueType))),
@@ -865,8 +831,7 @@ class X509V3ExtenstionsType(_STIXBase):
     """
 
     _type = 'x509-v3-extensions-type'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('basic_constraints', StringProperty()),
         ('name_constraints', StringProperty()),
         ('policy_constraints', StringProperty()),
@@ -893,8 +858,7 @@ class X509Certificate(_Observable):
     """
 
     _type = 'x509-certificate'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('is_self_signed', BooleanProperty()),
         ('hashes', HashesProperty()),
@@ -1038,8 +1002,7 @@ def CustomObservable(type='x-custom-observable', properties=None):
                 raise ValueError("Invalid observable type name '%s': must be between 3 and 250 characters." % type)
 
             _type = type
-            _properties = OrderedDict()
-            _properties.update([
+            _properties = OrderedDict([
                 ('type', TypeProperty(_type)),
             ])
 
@@ -1117,12 +1080,11 @@ def CustomExtension(observable=None, type='x-custom-observable', properties=None
                 raise ValueError("Invalid extension type name '%s': must be between 3 and 250 characters." % type)
 
             _type = type
-            _properties = OrderedDict()
 
             if not properties or not isinstance(properties, list):
                 raise ValueError("Must supply a list, containing tuples. For example, [('property1', IntegerProperty())]")
 
-            _properties.update(properties)
+            _properties = OrderedDict(properties)
 
             def __init__(self, **kwargs):
                 _Extension.__init__(self, **kwargs)
diff --git a/stix2/v21/sdo.py b/stix2/v21/sdo.py
index 98b40fa..743e788 100644
--- a/stix2/v21/sdo.py
+++ b/stix2/v21/sdo.py
@@ -27,11 +27,10 @@ class AttackPattern(STIXDomainObject):
     """
 
     _type = 'attack-pattern'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -42,7 +41,7 @@ class AttackPattern(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -54,11 +53,10 @@ class Campaign(STIXDomainObject):
     """
 
     _type = 'campaign'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -72,7 +70,7 @@ class Campaign(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -84,11 +82,10 @@ class CourseOfAction(STIXDomainObject):
     """
 
     _type = 'course-of-action'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -98,7 +95,7 @@ class CourseOfAction(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -110,11 +107,10 @@ class Identity(STIXDomainObject):
     """
 
     _type = 'identity'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -127,7 +123,7 @@ class Identity(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -139,11 +135,10 @@ class Indicator(STIXDomainObject):
     """
 
     _type = 'indicator'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty()),
@@ -157,7 +152,7 @@ class Indicator(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -169,11 +164,10 @@ class IntrusionSet(STIXDomainObject):
     """
 
     _type = 'intrusion-set'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -190,7 +184,7 @@ class IntrusionSet(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -202,12 +196,11 @@ class Location(STIXDomainObject):
     """
 
     _type = 'location'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('spec_version', StringProperty(fixed='2.1')),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('description', StringProperty()),
@@ -225,15 +218,14 @@ class Location(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
 
 class AnalysisType(_STIXBase):
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('start_time', TimestampProperty()),
         ('end_time', TimestampProperty()),
         ('analysis_tools', ObservableProperty()),
@@ -244,8 +236,7 @@ class AnalysisType(_STIXBase):
 
 class AVResultsType(_STIXBase):
 
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('product', StringProperty()),
         ('engine_version', StringProperty()),
         ('definition_version', StringProperty()),
@@ -263,12 +254,11 @@ class Malware(STIXDomainObject):
     """
 
     _type = 'malware'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('spec_version', StringProperty(fixed='2.1')),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -279,7 +269,7 @@ class Malware(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
         ('is_family', BooleanProperty(required=True)),
         ('first_seen', TimestampProperty()),
@@ -302,12 +292,11 @@ class Note(STIXDomainObject):
     """
 
     _type = 'note'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('spec_version', StringProperty(fixed='2.1')),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('summary', StringProperty()),
@@ -319,7 +308,7 @@ class Note(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -331,11 +320,10 @@ class ObservedData(STIXDomainObject):
     """
 
     _type = 'observed-data'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('first_observed', TimestampProperty(required=True)),
@@ -347,7 +335,7 @@ class ObservedData(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -365,12 +353,11 @@ class Opinion(STIXDomainObject):
     """
 
     _type = 'opinion'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('spec_version', StringProperty(fixed='2.1')),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('description', StringProperty()),
@@ -388,7 +375,7 @@ class Opinion(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -400,11 +387,10 @@ class Report(STIXDomainObject):
     """
 
     _type = 'report'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -416,7 +402,7 @@ class Report(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -428,11 +414,10 @@ class ThreatActor(STIXDomainObject):
     """
 
     _type = 'threat-actor'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -450,7 +435,7 @@ class ThreatActor(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -462,11 +447,10 @@ class Tool(STIXDomainObject):
     """
 
     _type = 'tool'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -478,7 +462,7 @@ class Tool(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -490,11 +474,10 @@ class Vulnerability(STIXDomainObject):
     """
 
     _type = 'vulnerability'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('name', StringProperty(required=True)),
@@ -504,7 +487,7 @@ class Vulnerability(STIXDomainObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -545,11 +528,10 @@ def CustomObject(type='x-custom-type', properties=None):
                 raise ValueError("Invalid type name '%s': must be between 3 and 250 characters." % type)
 
             _type = type
-            _properties = OrderedDict()
-            _properties.update([
+            _properties = OrderedDict([
                 ('type', TypeProperty(_type)),
                 ('id', IDProperty(_type)),
-                ('created_by_ref', ReferenceProperty(type="identity")),
+                ('created_by_ref', ReferenceProperty(type='identity')),
                 ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
                 ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
             ])
@@ -557,7 +539,7 @@ def CustomObject(type='x-custom-type', properties=None):
             if not properties or not isinstance(properties, list):
                 raise ValueError("Must supply a list, containing tuples. For example, [('property1', IntegerProperty())]")
 
-            _properties.update([x for x in properties if not x[0].startswith("x_")])
+            _properties.update([x for x in properties if not x[0].startswith('x_')])
 
             # This is to follow the general properties structure.
             _properties.update([
@@ -566,12 +548,12 @@ def CustomObject(type='x-custom-type', properties=None):
                 ('confidence', IntegerProperty()),
                 ('lang', StringProperty()),
                 ('external_references', ListProperty(ExternalReference)),
-                ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+                ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
                 ('granular_markings', ListProperty(GranularMarking)),
             ])
 
             # Put all custom properties at the bottom, sorted alphabetically.
-            _properties.update(sorted([x for x in properties if x[0].startswith("x_")], key=lambda x: x[0]))
+            _properties.update(sorted([x for x in properties if x[0].startswith('x_')], key=lambda x: x[0]))
 
             def __init__(self, **kwargs):
                 _STIXBase.__init__(self, **kwargs)
@@ -584,7 +566,7 @@ def CustomObject(type='x-custom-type', properties=None):
                         return
                     raise e
 
-        _register_type(_Custom, version="2.1")
+        _register_type(_Custom, version='2.1')
         return _Custom
 
     return custom_builder
diff --git a/stix2/v21/sro.py b/stix2/v21/sro.py
index 1440ebe..3d29b62 100644
--- a/stix2/v21/sro.py
+++ b/stix2/v21/sro.py
@@ -22,12 +22,11 @@ class Relationship(STIXRelationshipObject):
     """
 
     _type = 'relationship'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
-        ('spec_version', StringProperty(fixed="2.1")),
+        ('spec_version', StringProperty(fixed='2.1')),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('relationship_type', StringProperty(required=True)),
@@ -39,7 +38,7 @@ class Relationship(STIXRelationshipObject):
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
@@ -64,26 +63,25 @@ class Sighting(STIXRelationshipObject):
     """
 
     _type = 'sighting'
-    _properties = OrderedDict()
-    _properties.update([
+    _properties = OrderedDict([
         ('type', TypeProperty(_type)),
         ('id', IDProperty(_type)),
-        ('created_by_ref', ReferenceProperty(type="identity")),
+        ('created_by_ref', ReferenceProperty(type='identity')),
         ('created', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('modified', TimestampProperty(default=lambda: NOW, precision='millisecond')),
         ('first_seen', TimestampProperty()),
         ('last_seen', TimestampProperty()),
         ('count', IntegerProperty()),
         ('sighting_of_ref', ReferenceProperty(required=True)),
-        ('observed_data_refs', ListProperty(ReferenceProperty(type="observed-data"))),
-        ('where_sighted_refs', ListProperty(ReferenceProperty(type="identity"))),
+        ('observed_data_refs', ListProperty(ReferenceProperty(type='observed-data'))),
+        ('where_sighted_refs', ListProperty(ReferenceProperty(type='identity'))),
         ('summary', BooleanProperty()),
         ('revoked', BooleanProperty(default=lambda: False)),
         ('labels', ListProperty(StringProperty)),
         ('confidence', IntegerProperty()),
         ('lang', StringProperty()),
         ('external_references', ListProperty(ExternalReference)),
-        ('object_marking_refs', ListProperty(ReferenceProperty(type="marking-definition"))),
+        ('object_marking_refs', ListProperty(ReferenceProperty(type='marking-definition'))),
         ('granular_markings', ListProperty(GranularMarking)),
     ])
 
diff --git a/stix2/version.py b/stix2/version.py
index 7863915..a6221b3 100644
--- a/stix2/version.py
+++ b/stix2/version.py
@@ -1 +1 @@
-__version__ = "1.0.2"
+__version__ = '1.0.2'