C implementation of ZoneInfo cannot be subclassed

[sdispater]

While working on supporting zoneinfo in pendulum, I got this error whenever I subclassed Zoneinfo:

TypeError: descriptor '__init_subclass__' of 'backports.zoneinfo.ZoneInfo' object needs an argument

When using the zoneinfo module available in Python 3.9, I got this error:

TypeError: unbound method ZoneInfo.__init_subclass__() needs an argument

Note that there is no issue with the pure Python implementation.

So, I went digging and it appears that, at the C level, the __init_subclass__ / zoneinfo_init_subclass must be declared as a class method via METH_CLASS here: https://github.com/pganssle/zoneinfo/blob/master/lib/zoneinfo_module.c#L2584

I tested this quickly and it seems to fix the issue.

The fix should be applied on the backport and in the standard library.

[pganssle]

Huh... I can reproduce this with:

from backports.zoneinfo import ZoneInfo

class ZoneInfoSubclass(ZoneInfo):
    pass

But what I don't understand is why this test is not affected.

I guess whatever is wrong is making it so the C __init_subclass__ is never getting called, since that's never hit in the C coverage.

[pganssle]

Ah, I see the problem with the test. It's not inheriting from ZoneInfoSubclassTest. 😅

Fix coming up.