iter() not working with datetime.now()
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 partialj = 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.
I assume you're using CPython and not another Python implementation. And I can reproduce the issue with CPython 3.6.1 (I don't have PyPy, Jython, IronPython, ... so I can't check these).
The offender in this case is the replacement of PyObject_Call
with _PyObject_CallNoArg
in the C equivalent of the callable_iterator.__next__
(your object is a callable_iterator
) method.
The PyObject_Call
does return a new datetime.datetime
instance while _PyObject_CallNoArg
returns NULL
(which is roughly equivalent to an exception in Python).
Digging a bit through the CPython source code:
The _PyObject_CallNoArg
is just a macro for _PyObject_FastCall
which in turn is a macro for _PyObject_FastCallDict
.
This _PyObject_FastCallDict
function checks the type of the function (C
-function or Python function or something else) and delegates to _PyCFunction_FastCallDict
in this case because datetime.now
is a C function.
Since datetime.datetime.now
has the METH_FASTCALL
flag it ends up in the fourth case
but there _PyStack_UnpackDict
returns NULL
and the function is never even called.
I'll stop there and let the Python devs figure out what's wrong in there. @Martijn Pieters already filed a Bug report and they will fix it (I just hope they fix it soonish).
So it's a Bug they introduced in 3.6 and until it's fixed you need to make sure the method isn't a CFunction
with the METH_FASTCALL
flag. As workaround you can wrap it. Apart from the possibilities @Martijn Pieters mentioned there is also a simple:
def now(): return datetime.datetime.now()j = iter(now, None)next(j) # datetime.datetime(2017, 5, 31, 14, 23, 1, 95999)