[Python-Dev] PySequence_Check but no __len__

[Brett Cannon]

Sorry, I don't quite follow.

On Thu, 21 Jun 2018 at 08:50 Christian Tismer <tismer at stackless.com> wrote:

> Hi friends,
>
> there is a case in the Python API where I am not sure what to do:
>
> If an object defines __getitem__() only but no __len__(),
> then PySequence_Check() already is true and does not care.
>

Which matches
https://docs.python.org/3/c-api/sequence.html#c.PySequence_Check .

>From Objects/abstract.c:

int
PySequence_Check(PyObject *s)
{
    if (PyDict_Check(s))
        return 0;
    return s != NULL && s->ob_type->tp_as_sequence &&
        s->ob_type->tp_as_sequence->sq_item != NULL;
}



>
> So if I define no __len__, it simply fails. Is this intended?
>

What is "it" in this case that is failing? It isn't PySequence_Check() so
I'm not sure what the issue is.

-Brett


>
> I was mislead and thought this was the unlimited case, but
> it seems still to be true that sequences are always finite.
>
> Can someone please enlighten me?
> --
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