[Python-Dev] datetime.fromisocalendar

Victor Stinner vstinner at redhat.com
Mon Apr 29 09:30:41 EDT 2019


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" !?!

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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