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 a écrit :

I think it’s a good idea.

On Sat, Apr 27, 2019 at 11:43 AM Paul Ganssle 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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