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

[Guido van Rossum]

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

[Yahya Abou 'Imran via Python-ideas]

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 

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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