[issue40799] Create Lib/_pydatetime.py file to optimize "import datetime" when _datetime is available

[STINNER Victor]

STINNER Victor <vstinner at python.org> added the comment:

> I believe that there are other modules that have similar situations like heapq, but datetime is probably the worst offender.

heapq seems to be a little bit different. _heapq is not a drop-in replacement of heapq.py. For example, nlargest() seems to only be implemented in pure Python.


> I am inclined to say that we should restructure datetime into a folder, containing __init__.py, _datetime.py and possibly _strptime.py (which I think is also only used in datetime), but I think that sort of restructuring is way more sensitive to weird import bugs than this one.

I have no idea what are the side effects of converting datetime.py file into a package.

A single file _pydatetime.py seems more convenient to me. I'm aware of _strptime.py but I don't see it as a datetime submodule and I don't see the value of moving it as a datetime submodule.

I'm fine with _datetime accessing _strptime module. It sounds more complex to me if _datetime would be imported by datetime which contains datetime._strptime. I see a higher risk of subtle import issues, since datetime has two implementations (C and Python). But it may be wrong :-)

Also, all other stdlib modules which have a C implementation are designed with files, not folders: io.py (_io and _pyio) and decimal.py (_decimal and _pydecimal) are good examples.

I mostly case about reducing the number of indirect imports and import performance. I don't have a strong opinion about file vs folder.


> As it is now, I would be shocked if this didn't break *someone*, because people are always relying on weird implementation details (knowingly or unknowingly), but I think it's worth doing; it's good to tackle it this early in the cycle.

I'm fine with breaking applications relying on implementation details. Also, we can adjust the code to fix such corner cases later if it's needed, possible and justified :-)

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