Vedran Čačić added the comment:
Yes, the mapping/sequence distinction was (at least declaratively) the reason
the ABCs were introduced, but that isn't an obstacle here: whether a mapping or
a sequence, it _is_ iterable, right?
---
In case anybody is interested, here's how I came to this problem: at a
programming competition, I set a problem where contestants had to write some
function, and I declared that "the function must work for arbitrary iterable
(with some properties that currently don't matter)".
Then a big discussion ensued, with a big group of people thinking that classes
with __getitem__ but no __iter__ don't quality (giving collections.abc.Iterable
as an argument), and another big group of people thinking they do (giving EAFP
as an argument: "look, I tried iterating, and succeeded").
Of course, it's an incredibly technical detail, but I don't like such gray
areas. To me, things with __getitem__ are clearly iterable - the glossary says
so:-). Iterable's instancecheck is simply buggy ("incomplete", if you want).
There might be valid reasons for keeping it buggy, but they should be
documented.
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<http://bugs.python.org/issue18558>
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