models: Make sure sub-classes can extend models

mopidy/models/immutable.py

@@ -22,7 +22,8 @@ class ImmutableObjectMeta(type):
 
         attrs['_fields'] = fields
         attrs['_instances'] = weakref.WeakValueDictionary()
-        attrs['__slots__'] = ['_hash'] + fields.values()
+        attrs['__slots__'] = list(attrs.get('__slots__', []))
+        attrs['__slots__'].extend(['_hash'] + fields.values())
 
         for ancestor in [b for base in bases for b in inspect.getmro(base)]:
             if '__weakref__' in getattr(ancestor, '__slots__', []):

tests/models/test_models.py

@@ -18,6 +18,14 @@ class InheritanceTest(unittest.TestCase):
         class Foo(Track):
             pass
 
+    def test_sub_class_can_have_its_own_slots(self):
+
+        class Foo(Track):
+            __slots__ = ('_foo',)
+
+        f = Foo()
+        f._foo = 123
+
 
 class CachingTest(unittest.TestCase):