[pypy-issue] Issue #2635: datetime.replace always returns datetime.datetime in pypy3 (pypy/pypy)

[Paul G]

New issue 2635: datetime.replace always returns datetime.datetime in pypy3
https://bitbucket.org/pypy/pypy/issues/2635/datetimereplace-always-returns

Paul G:

MWE:


`demo_datetime_replace.py`:

```
#!python

from datetime import datetime

class SuperDatetime(datetime):
    pass

if __name__ == "__main__":
    sdt0 = SuperDatetime(2017, 8, 1)
    sdt1 = sdt0.replace(2016, 8, 1)

    print(repr(sdt0))
    print(repr(sdt1))
```

```
#! bash
$ python2 demo_datetime_replace.py
SuperDatetime(2017, 8, 1, 0, 0)
SuperDatetime(2016, 8, 1, 0, 0)

$ python3 demo_datetime_replace.py 
SuperDatetime(2017, 8, 1, 0, 0)
SuperDatetime(2016, 8, 1, 0, 0)

$ pypy demo_datetime_replace.py
SuperDatetime(2017, 8, 1, 0, 0)
SuperDatetime(2016, 8, 1, 0, 0)

$ pypy3 demo_datetime_replace.py 
__main__.SuperDatetime(2017, 8, 1, 0, 0)
datetime.datetime(2016, 8, 1, 0, 0)
```

It seems that `pypy3` is *also* the odd man out with respect to how `repr` is treated, but that's a separate and less important issue.

It's not clear to me how much "`datetime.replace` returns the same class of object" is part of the Python spec, but it seems reasonable and it's definitely a shift in behavior. There's [something about subclasses of builtin types here](http://doc.pypy.org/en/latest/cpython_differences.html#subclasses-of-built-in-types), but it doesn't seem relevant to me, if I'm understanding it right.