iter() not working with datetime.now()

Question

A simple snippet in Python 3.6.1:

import datetime j = iter(datetime.datetime.now, None) next(j) 

returns:

Traceback (most recent call last):   File "<stdin>", line 1, in <module> StopIteration 

instead of printing out the classic now() behavior with each next().

I've seen similar code working in Python 3.3, am I missing something or has something changed in version 3.6.1?

Answer 1

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

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

from functools import partial  j = iter(partial(datetime.datetime.now), 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.