[Python-Dev] PySequence_Check but no __len__

[Paul Moore]

On 22 June 2018 at 20:17, Ivan Pozdeev via Python-Dev
<python-dev at python.org> wrote:
> On 22.06.2018 22:07, Terry Reedy wrote:

>> https://docs.python.org/3/library/collections.abc.html#collections-abstract-base-classes
>>
>> says that a Sequence has both __getitem__ and __len__.
>>
>> I am surprised that a C-API function calls something a 'sequence' without
>> it having __len__.
>>
> A practical sequence check is checking for __iter__ . An iterator doesn't
> necessarily have a defined length -- e.g. a stream or a generator.

There's a difference between the ABC "Sequence" and the informally
named sequence concept used in the C API. It's basically just that the
C API term predates the ABC significantly, and there's no way that
we'd change the C API naming because it would break too much code, but
IMO it's just one of those "historical reasons" type of things that
can't really be adequately explained, but just needs to be accepted...

An ABC Sequence has __getitem__ and __len__. In terms of ABCs,
something with __iter__ is an Iterable. Informal terminology is a
different matter...

Paul