Mailman 3 Make MappingView inherit from Collection instead of Sized - Python-ideas

Dec. 29, 2017

      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?

Make MappingView inherit from Collection instead of Sized

Yahya Abou 'Imran

Guido van Rossum

Guido van Rossum

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