[Python-Dev] datetime.fromisocalendar (original) (raw)

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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