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[Python-ideas] Make MappingView inherit from Collection instead of Sized

Guido van Rossum gvanrossum at gmail.com
Fri Dec 29 14:11:57 EST 2017 
This sounds like a good observation. I recommend opening a bug and
preparing a PR if you can (a PR would also help finding if there are any
problems with the idea).

On Dec 29, 2017 9:50 AM, "Yahya Abou 'Imran via Python-ideas" <
python-ideas at python.org> wrote:

> After I generate an UML diagram from collections.abc, I found very strange
> that MappingView inherit from Sized instead of Collection (new in python
> 3.6).
>
> Yes, MappingView only define __len__ and not __iter__ and __contains__,
> but all of its subclasses define them (KeysView, ValuesView and ItemViews).
>
> I tried to run the tests in test/test_collections.py after making this
> change and on only one fail :
>
> Traceback (most recent call last):
>   File "/usr/lib/python3.6/test/test_collections.py", line 789, in
> test_Collection
>     self.assertNotIsInstance(x, Collection)
> AssertionError: dict_values([]) is an instance of <class
> 'collections.abc.Collection'>
>
> Wich is absolutely wrong, since in reality a dict_values instance has the
> behaviour of a Collection:
>
> >>> vals = {1:'a', 2: 'b'}.values()
> >>> 'a' in vals
> True
> >>> 'c' in vals
> False
> >>> len(vals)
> 2
> >>> for val in vals:
> ...     print(val)
> ...
> a
> b
>
> The only lack is that it doesn't define a __contains__ method:
>
> >>> '__contains__' in vals
> False
>
> It uses __iter__ to find the presence of the value.
>
> But, hey: we have register() for this cases! In fact, when MappingView
> inherit from Collection, dict_values is considered as a subclass of
> Collection since it's in the register of ValuesView, causing the above
> bug...
> So, the test have to be changed, and dict_values must be placed in the
> samples that pass the test, and not in the ones that fail it.
>
> Maybe we can open an issue on the python bug tracker?
>
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