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

Brett Cannon brett at python.org
Tue Jun 14 16:44:19 EDT 2016

On Tue, 14 Jun 2016 at 13:30 Alan Franzoni <mailing at franzoni.eu> wrote:

> 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?

To add some extra info, both KeysView and ItemsView inherit from Set which
does inherit from Iterable. I personally don't know why ValuesView doesn't
inherit from Set (although Iterable does override __subclasshook__() so
there isn't a direct functional loss which if this turns out to be a bug
why no one has notified until now).

Alan, would you mind filing an issue at bugs.python.org about this?
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