Paul Ganssle <p.gans...@gmail.com> added the comment:
I can reproduce this on Linux with Python 3.8.2.
I think this may be a bug, but it may also just be platform-specific weirdness.
Either way it's very curious behavior:
>>> datetime.strptime("2023-0-0", "%Y-%W-%w")
datetime.datetime(2023, 1, 1, 0, 0)
>>> datetime.strptime("2023-0-1", "%Y-%W-%w")
datetime.datetime(2022, 12, 26, 0, 0)
The definition for %W (and %U, which is related) goes like this:
> Week number of the year (Monday as the first day of the week) as a decimal
> number. All days in a new year preceding the first Monday are considered to
> be in week 0.
2024 starts on a Monday, so there should be no Week 0 in that year at all.
Seems to me like it's undefined what happens when you put in a string that puts
in an invalid value for "%Y-%W-%w".
Seems to me that we are just passing through the behavior of `time.strptime` in
this case (which just calls out to what the platform does):
>>> time.strptime("2024-0-3", "%Y-%W-%w")
time.struct_time(tm_year=2024, tm_mon=1, tm_mday=3, tm_hour=0, tm_min=0,
tm_sec=0, tm_wday=2, tm_yday=3, tm_isdst=-1)
I am open to discussion about trying to rationalize this behavior - it would be
a bit tricky but if we moved to our own implementation of the algorithm to
calculate %W we could detect this situation and throw an exception. I'd rather
see if this is intended behavior in the underlying C implementation first,
though. If this is consistent across platforms and not just some random
implementation detail, people may be relying on it.
I propose that we:
1. Determine what happens on different platforms (might be easy to just make a
PR that asserts the current behavior and see if/how it breaks on any of the
supported platforms).
2. Determine why it works the way it does.
After that, at the very least we should document the behavior with a warning or
a footnote or something. If we make any changes to the behavior they would be
3.9+, but the documentation changes can be backported.
Thanks for the bug report zhanying! Very interesting!
----------
versions: +Python 3.7, Python 3.8, Python 3.9
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<https://bugs.python.org/issue40236>
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