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

Alan Franzoni mailing at franzoni.eu
Tue Jun 14 16:25:24 EDT 2016

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,

-- www.franzoni.eu - Twitter: @alanfranz contact me at public@[mysurname].eu

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