In the .py implementation of datetime.replace (and date.replace and time.replace), the new datetime is created using the datetime type:

https://github.com/python/cpython/blob/master/Lib/datetime.py#L1578

But in the C source, it is created from type(self):

https://github.com/python/cpython/blob/master/Modules/_datetimemodule.c#L5046

I think the second should be the preferred behavior, so the datetime.py source should be updated to reflect that it's calling self.__class__(...) rather than datetime(...). I can prepare a PR if this would be desirable.

(That said, I'm not 100% clear under what circumstances the code in datetime.py is actually *used*, so I'm not sure how to write tests for it - is datetime.py essentially documentation, or is there a way to explicitly fall back to it?)

Per this issue on the pypy3 tracker:

https://bitbucket.org/pypy/pypy/issues/2635/datetimereplace-always-returns