[gcdebug] Only ignore weakref.WeakKeyDictionary class on Python 2 From Python 3.5, this class fails isinstance check with the following error: :: cls = <class 'weakref.WeakKeyDictionary'>, instance = <functools._lru_list_elem object at 0x7f4331859a48> def __instancecheck__(cls, instance): """Override for isinstance(instance, cls).""" # Inline the cache checking > subclass = instance.__class__ E AssertionError: [<class 'AttributeError'> in gc] 'functools._lru_list_elem' object has no attribute '__class__' /usr/lib/python3.5/abc.py:181: AssertionError I have no clue why this happens, but it makes cubicweb.web.test.test_views.AutomaticWebTest.test_startup_views fail on Python 3.5. So only consider this class for Python 2.