getattr() checks for attribute membership, returns the value if not None and raises AttributeError when is not present. fixes #19

stix2/base.py

@@ -78,7 +78,7 @@ class _STIXBase(collections.Mapping):
             raise DependentPropertiesError(self.__class__, failed_dependency_pairs)
 
     def _check_object_constraints(self):
-        if self.granular_markings:
+        if hasattr(self, "granular_markings") and self.granular_markings:
             for m in self.granular_markings:
                 # TODO: check selectors
                 pass
@@ -124,7 +124,16 @@ class _STIXBase(collections.Mapping):
 
     # Handle attribute access just like key access
     def __getattr__(self, name):
-        return self.get(name)
+        # If the requested attribute is not None return its value
+        if self.get(name) is not None:
+            return self.get(name)
+        # Otherwise check the attribute exists inside object. If it exists
+        # return None. If not raise the AttributeError.
+        elif name in self._properties:
+            return None
+
+        raise AttributeError("'%s' object has no attribute '%s'" %
+                             (self.__class__.__name__, name))
 
     def __setattr__(self, name, value):
         if name != '_inner' and not name.startswith("_STIXBase__"):