[issue7989] Add pure Python implementation of datetime module to CPython

[Alexander Belopolsky]

Alexander Belopolsky <belopolsky at users.sourceforge.net> added the comment:

> I am talking specifically about this kind of assert:
> 
>    assert 1 <= month <= 12, 'month must be in 1..12'
>
> I think it should be replaced with:
>
>    if month < 1 or month > 12:
>        raise ValueError('month must be in 1..12')

I reviewed the asserts.  Value range checking asserts appear in non-public functions which are not called with out-of-range values by the module code.  Therefore they can only be triggered if there is a bug in the future version of datetime.py.  This is expressly what asserts are for.

There is another type of asserts that should be either removed or modified:

assert daysecondswhole == int(daysecondswhole)  # can't overflow   

Since int is long in 3.x, this assert does not check anything. We can replace this with

assert daysecondswhole.bit_length() <= 32

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue7989>
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