iter() not working with

This is definitely a bug introduced in Python 3.6.0b1. The iter() implementation recently switched to using _PyObject_FastCall() (an optimisation, see issue 27128), and it must be this call that is breaking this.

The same issue arrises with other C classmethod methods backed by Argument Clinic parsing:

>>> from asyncio import Task
>>> Task.all_tasks()
>>> next(iter(Task.all_tasks, None))
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>

If you need a work-around, wrap the callable in a functools.partial() object:

from functools import partial

j = iter(partial(, None)

I filed issue 30524 — iter(classmethod, sentinel) broken for Argument Clinic class methods? with the Python project. The fix for this has landed and is part of 3.6.2rc1.

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