models: Make sure we really only add __weakref__ once

mopidy/models.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import, unicode_literals
 
 import copy
+import inspect
 import json
 import weakref
 
@@ -145,11 +146,12 @@ class ImmutableObjectMeta(type):
                 value._name = key
 
         attrs['_fields'] = fields
-        attrs['__slots__'] = fields.values()
         attrs['_instances'] = weakref.WeakValueDictionary()
+        attrs['__slots__'] = fields.values()
 
-        for base in bases:
+        anncestors = [b for base in bases for b in inspect.getmro(base)]
-            if '__weakref__' in getattr(base, '__slots__', []):
+        for anncestor in anncestors:
+            if '__weakref__' in getattr(anncestor, '__slots__', []):
                 break
         else:
             attrs['__slots__'].append('__weakref__')

tests/models/test_models.py

@@ -8,6 +8,17 @@ from mopidy.models import (
     TlTrack, Track, model_json_decoder)
 
 
+class InheritanecTest(unittest.TestCase):
+
+    def test_weakref_and_slots_play_nice_in_subclass(self):
+        # Check that the following does not happen:
+        # TypeError: Error when calling the metaclass bases
+        #   __weakref__ slot disallowed: either we already got one...
+
+        class Foo(Track):
+            pass
+
+
 class CachingTest(unittest.TestCase):
 
     def test_same_instance(self):