[Python-Dev] datetime.fromisocalendar

[Paul Ganssle]

Greetings,

Some time ago, I proposed adding a `.fromisocalendar` alternate
constructor to `datetime` (bpo-36004
<https://bugs.python.org/issue36004>), with a corresponding
implementation (PR #11888
<https://github.com/python/cpython/pull/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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