Code style and behavioral improvements in ExtensionsProperty:

- Changed an error message "Cannot determine extension type."
At that point in the code, we in fact have a registered
extension type and the class for it, so it didn't seem to make
any sense.  I changed it to say that an extension of that type
couldn't be created from type of value given.  Because this is
really about typing (in both the old and new code), I also
changed the exception class to TypeError.

- Changed customization behavior: unregistered
"extension-definition--" style extensions are no longer
considered custom.  Their mere presence can no longer be
considered a customization, since it doesn't fit the
criteria, and in fact the extension mechanism is supposed to
supplant the old customization system anyway.  Unregistered
extensions of other types are still considered custom.

- Fixed up unit tests according to the exception message
change.

- Added a unit test for unregistered "extension-definition--"
style extensions.

stix2/properties.py

@@ -14,7 +14,7 @@ import stix2.hashes
 from .base import _STIXBase
 from .exceptions import CustomContentError, DictionaryKeyError, STIXError
 from .parsing import parse, parse_observable
-from .registry import STIX2_OBJ_MAPS
+from .registry import class_for_type
 from .utils import (
     STIXTypeClass, _get_dict, get_class_hierarchy_names, get_type_from_id,
     is_object, is_stix_type, parse_into_datetime, to_enum,
@@ -781,17 +781,21 @@ class ExtensionsProperty(DictionaryProperty):
             raise ValueError("The extensions property must contain a dictionary")
 
         has_custom = False
-        extension_type_map = STIX2_OBJ_MAPS[self.spec_version].get('extensions', {})
         for key, subvalue in dictified.items():
-            if key in extension_type_map:
-                cls = extension_type_map[key]
-                if type(subvalue) is dict:
+            cls = class_for_type(key, self.spec_version, "extensions")
+            if cls:
+                if isinstance(subvalue, dict):
                     ext = cls(allow_custom=allow_custom, **subvalue)
-                elif type(subvalue) is cls:
-                    # If already an instance of an _Extension class, assume it's valid
+                elif isinstance(subvalue, cls):
+                    # If already an instance of the registered class, assume
+                    # it's valid
                     ext = subvalue
                 else:
-                    raise ValueError("Cannot determine extension type.")
+                    raise TypeError(
+                        "Can't create extension '{}' from {}.".format(
+                            key, type(subvalue)
+                        )
+                    )
 
                 has_custom = has_custom or ext.has_custom
 
@@ -805,15 +809,24 @@ class ExtensionsProperty(DictionaryProperty):
                 dictified[key] = ext
 
             else:
-                if allow_custom:
+                # If an unregistered "extension-definition--" style extension,
+                # we don't know what's supposed to be in it, so we can't
+                # determine whether there's anything custom.  So, assume there
+                # are no customizations.  If it's a different type of extension,
+                # non-registration implies customization (since all spec-defined
+                # extensions should be pre-registered with the library).
+
+                if key.startswith('extension-definition--'):
+                    _validate_id(
+                        key, self.spec_version, 'extension-definition--'
+                    )
+                elif allow_custom:
                     has_custom = True
-                    dictified[key] = subvalue
-                elif key.startswith('extension-definition--'):
-                    _validate_id(key, '2.1', 'extension-definition')
-                    dictified[key] = subvalue
                 else:
                     raise CustomContentError("Can't parse unknown extension type: {}".format(key))
 
+                dictified[key] = subvalue
+
         return dictified, has_custom
 
 

stix2/test/v20/test_custom.py

@@ -795,7 +795,7 @@ def test_custom_extension_wrong_observable_type():
             },
         )
 
-    assert 'Cannot determine extension type' in excinfo.value.reason
+    assert "Can't create extension 'ntfs-ext' from" in excinfo.value.reason
 
 
 @pytest.mark.parametrize(

stix2/test/v21/test_custom.py

@@ -984,7 +984,7 @@ def test_custom_extension_wrong_observable_type():
             },
         )
 
-    assert 'Cannot determine extension type' in excinfo.value.reason
+    assert "Can't create extension 'ntfs-ext'" in excinfo.value.reason
 
 
 @pytest.mark.parametrize(
@@ -1228,6 +1228,41 @@ def test_parse_observable_with_unregistered_custom_extension(data):
     assert not isinstance(parsed_ob['extensions']['x-foobar-ext'], stix2.base._STIXBase)
 
 
+def test_unregistered_new_style_extension():
+
+    f_dict = {
+        "type": "file",
+        "name": "foo.txt",
+        "extensions": {
+            "extension-definition--31adb724-a9a4-44b6-8ec2-fd4b181c9507": {
+                "extension-type": "property-extension",
+                "a": 1,
+                "b": True
+            }
+        }
+    }
+
+    f = stix2.parse(f_dict, allow_custom=False)
+
+    assert f.extensions[
+        "extension-definition--31adb724-a9a4-44b6-8ec2-fd4b181c9507"
+    ]["a"] == 1
+    assert f.extensions[
+        "extension-definition--31adb724-a9a4-44b6-8ec2-fd4b181c9507"
+    ]["b"]
+    assert not f.has_custom
+
+    f = stix2.parse(f_dict, allow_custom=True)
+
+    assert f.extensions[
+        "extension-definition--31adb724-a9a4-44b6-8ec2-fd4b181c9507"
+    ]["a"] == 1
+    assert f.extensions[
+        "extension-definition--31adb724-a9a4-44b6-8ec2-fd4b181c9507"
+    ]["b"]
+    assert not f.has_custom
+
+
 def test_register_custom_object():
     # Not the way to register custom object.
     class CustomObject2(object):