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Tue Feb 12 12:38:48 CET 2013 · https://mail.python.org/pipermail/python-dev/2013-February/123979.html

2013/2/12 Steven D'Aprano <steve at 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
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Showing content from https://mail.python.org/pipermail/python-dev/2013-February/123979.html below:

[Python-Dev] what is a dict_keys and where can I import it from?