[Python-Dev] datetime.fromisocalendar

[Ivan Pozdeev]

On 29.04.2019 16:30, Victor Stinner wrote:
> I reviewed and merged Paul's PR. I concur with Guido, the new
> constructor perfectly makes sense and is useful.
>
> About the implementation: date and time are crazy beasts. Extract of the code:
>
>          if not 0 < week < 53:
>              out_of_range = True
>
>              if week == 53:
>                  # ISO years have 53 weeks in them on years starting with a
>                  # Thursday and leap years starting on a Wednesday
>                  first_weekday = _ymd2ord(year, 1, 1) % 7
>                  if (first_weekday == 4 or (first_weekday == 3 and
>                                             _is_leap(year))):
>                      out_of_range = False
>
>              if out_of_range:
>                  raise ValueError(f"Invalid week: {week}")
>
> "ISO years have 53 weeks in them on years starting with a Thursday and
> leap years starting on a Wednesday" !?!
https://www.staff.science.uu.nl/~gent0113/calendar/isocalendar.htm , linked from 
https://docs.python.org/3/library/datetime.html?highlight=isocalendar#datetime.date.isocalendar
> Victor
>
> Le sam. 27 avr. 2019 à 22:37, Guido van Rossum <guido at python.org> a écrit :
>> I think it’s a good idea.
>>
>> On Sat, Apr 27, 2019 at 11:43 AM Paul Ganssle <paul at ganssle.io> wrote:
>>> Greetings,
>>>
>>> Some time ago, I proposed adding a `.fromisocalendar` alternate constructor to `datetime` (bpo-36004), with a corresponding implementation (PR #11888). I advertised it on datetime-SIG some time ago but haven't seen much discussion there, so I'd like to bring it to python-dev's attention as we near the cut-off for new Python 3.8 features.
>>>
>>> Other than the fact that I've needed this functionality in the past, I also think a good general principle for the datetime module is that when a class (time, date, datetime) has a "serialization" method (.strftime, .timestamp, .isoformat, .isocalendar, etc), there should be a corresponding deserialization method (.strptime, .fromtimestamp, .fromisoformat) that constructs a datetime from the output. Now that `fromisoformat` was introduced in Python 3.7, I think `isocalendar` is the only remaining method without an inverse. Do people agree with this principle? Should we add the `fromisocalendar` method?
>>>
>>> Thanks,
>>> Paul
>>>
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>> --
>> --Guido (mobile)
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>
>
-- 
Regards,
Ivan