Add created_by() function to Environment

stix2/environment.py

@@ -152,3 +152,17 @@ class Environment(object):
     def parse(self, *args, **kwargs):
         return _parse(*args, **kwargs)
     parse.__doc__ = _parse.__doc__
+
+    def created_by(self, obj):
+        """Retrieve the Identity refered to by the object's `created_by_ref`.
+
+        Args:
+            obj: The STIX object whose `created_by_ref` property will be looked
+                up.
+
+        Returns:
+            The STIX object's creator.
+        """
+
+        creator_id = obj.get('created_by_ref', '')
+        return self.get(creator_id)

stix2/test/test_environment.py

@@ -184,3 +184,25 @@ def test_parse_malware():
     assert mal.modified == FAKE_TIME
     assert mal.labels == ['ransomware']
     assert mal.name == "Cryptolocker"
+
+
+def test_created_by():
+    identity = stix2.Identity(**IDENTITY_KWARGS)
+    factory = stix2.ObjectFactory(created_by_ref=identity.id)
+    env = stix2.Environment(store=stix2.MemoryStore(), factory=factory)
+    env.add(identity)
+
+    ind = env.create(stix2.Indicator, **INDICATOR_KWARGS)
+    creator = env.created_by(ind)
+    assert creator.id == identity.id
+
+
+def test_created_by_no_datasource():
+    identity = stix2.Identity(**IDENTITY_KWARGS)
+    factory = stix2.ObjectFactory(created_by_ref=identity.id)
+    env = stix2.Environment(factory=factory)
+
+    ind = env.create(stix2.Indicator, **INDICATOR_KWARGS)
+    with pytest.raises(AttributeError) as excinfo:
+        env.created_by(ind)
+    assert 'Environment has no data source' in str(excinfo.value)