[Python-Dev] ValuesView abc: why doesn't it (officially) inherit from Iterable?

[Alan Franzoni]

Hello,
I hope not to bother anyone with a somewhat trivial question, I was
unable to get an answer from other channels.

I was just checking out some docs on ABCs for a project of mine, where
I need to do some type-related work. Those are the official docs about
the ValuesView type, in both Python 2 and 3:

https://docs.python.org/2/library/collections.html#collections.ValuesView
https://docs.python.org/3/library/collections.abc.html

and this is the source (Python 2, but same happens in Python 3)

https://hg.python.org/releases/2.7.11/file/9213c70c67d2/Lib/_abcoll.py#l479

I was very puzzled about the ValuesView interface, because from a
logical standpoint it should inherit from Iterable, IMHO (it's even
got the __iter__ Mixin method); on the contrary the docs say that it
just inherits from MappingView, which inherits from Sized, which
doesn't inherit from Iterable.

So I fired up my 2.7 interpreter:

>>> from collections import Iterable
>>> d = {1:2, 3:4}
>>> isinstance(d.viewvalues(), Iterable)
True
>>>

It looks iterable, after all, because of Iterable's own subclasshook.

But I don't understand why ValuesView isn't explicitly Iterable. Other
ABCs, like Sequence, are explicitly inheriting Iterable. Is there some
arcane reason behind that, or it's just a documentation+implementation
shortcoming (with no real-world impact) for a little-used feature?

Bye,

-- 
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