add comment explaining attributeerror

synapse/events/__init__.py

@@ -47,6 +47,9 @@ class _EventInternalMetadata(object):
 
 
 def _event_dict_property(key):
+    # We want to be able to use hasattr with the event dict properties.
+    # However, (on python3) hasattr expects AttributeError to be raised. Hence,
+    # we need to transform the KeyError into an AttributeError
     def getter(self):
         try:
             return self._event_dict[key]