2013/2/12 Steven D'Aprano <steve@pearwood.info>
An anomaly, which I cannot explain:

py> issubclass(type(keys), KeysView)
True
py> type(keys) is KeysView
False
py> type(keys).__mro__
(<class 'dict_keys'>, <class 'object'>)


This disturbs my calm, because I expect that if issubclass returns True, the two classes will either be identical, or the second will be in the MRO of the first. What have I missed?

Ah, the magic of ABCs...
KeysView overrides __instancecheck__, and can pretend to be any other class.

This is precisely set in Lib/collections/abc.py:
  KeysView.register(dict_keys)


--
Amaury Forgeot d'Arc