diff --git a/setup.py b/setup.py
index a034bda..3a681fa 100644
--- a/setup.py
+++ b/setup.py
@@ -47,10 +47,10 @@ setup(
     keywords="stix stix2 json cti cyber threat intelligence",
     packages=find_packages(),
     install_requires=[
-        'pytz',
-        'six',
         'python-dateutil',
+        'pytz',
         'requests',
-        'simplejson'
+        'simplejson',
+        'six'
     ],
 )
diff --git a/stix2/sources/__init__.py b/stix2/sources/__init__.py
index c4a0f0f..6b8513e 100644
--- a/stix2/sources/__init__.py
+++ b/stix2/sources/__init__.py
@@ -529,7 +529,6 @@ class CompositeDataSource(object):
         """
         return copy.deepcopy(self.data_sources.values())
 
-
     def deduplicate(self, stix_obj_list):
         """Deduplicate a list of STIX objects to a unique set
 
diff --git a/stix2/sro.py b/stix2/sro.py
index 23f0dd0..af483bc 100644
--- a/stix2/sro.py
+++ b/stix2/sro.py
@@ -4,9 +4,9 @@ from collections import OrderedDict
 
 from .base import _STIXBase
 from .common import ExternalReference, GranularMarking
-from .properties import (BooleanProperty, IDProperty, IntegerProperty, ListProperty,
-                         ReferenceProperty, StringProperty, TimestampProperty,
-                         TypeProperty)
+from .properties import (BooleanProperty, IDProperty, IntegerProperty,
+                         ListProperty, ReferenceProperty, StringProperty,
+                         TimestampProperty, TypeProperty)
 from .utils import NOW
 
 
diff --git a/stix2/utils.py b/stix2/utils.py
index dc25799..aa0caa6 100644
--- a/stix2/utils.py
+++ b/stix2/utils.py
@@ -1,13 +1,13 @@
 """Utility functions and classes for the stix2 library."""
 
-from stix2 import base
-
 import datetime as dt
 import json
 
 from dateutil import parser
 import pytz
 
+from .base import _STIXBase
+
 # Sentinel value for properties that should be set to the current time.
 # We can't use the standard 'default' approach, since if there are multiple
 # timestamps in a single object, the timestamps will vary by a few microseconds.
@@ -132,7 +132,7 @@ def find_property_index(obj, properties, tuple_to_find):
         for pv in obj._inner.values():
             if isinstance(pv, list):
                 for item in pv:
-                    if isinstance(item, base._STIXBase):
+                    if isinstance(item, _STIXBase):
                         val = find_property_index(item,
                                                   item._object_properties(),
                                                   tuple_to_find)
@@ -145,7 +145,7 @@ def find_property_index(obj, properties, tuple_to_find):
                     except ValueError:
                         return len(tuple_to_find[0])
                 for item in pv.values():
-                    if isinstance(item, base._STIXBase):
+                    if isinstance(item, _STIXBase):
                         val = find_property_index(item,
                                                   item._object_properties(),
                                                   tuple_to_find)