3.12 regression: cannot create weak reference to 'typing.TypeVar' object

This works in Python 3.11, but fails in 3.12b3:

import weakref
import typing
weakref.ref(typing.TypeVar('T'))

I'm assuming it's an unintentional side effect of the PEP-695 implementation? Unfortunately I don't have time to investigate right now, let me know if I should.

Reported in cloudpipe/cloudpickle#507

Linked PRs

Fixing.

The new typing.TypeAliasType is also not weakrefable:

>>> type T = int
>>> weakref.ref(T)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create weak reference to 'typing.TypeAliasType' object

But neither is the existing UnionType:

>>> weakref.ref(int | str)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: cannot create weak reference to 'types.UnionType' object

So maybe that's fine?

pythongh-106403: Restore weakref support for TypeVar and friends

@JelleZijlstra

Thanks @JelleZijlstra!

So maybe that's fine?

I suspect that this might cause problems with cloudpickle when executing interactively defined functions with type annotations in their signatures on remote Python workers (e.g. in Dask, Ray or PySpark / databricks).

I did not have time to investigate the details myself yet but @musicinmybrain tried to test your fix in cloudpickle in cloudpipe/cloudpickle#507 (comment) and there are still segfaults to debug.

pythongh-106403: Restore weakref support for TypeVar and friends (pyt…

gh-106403: Restore weakref support for TypeVar and friends (#106418)

I merged my PR that re-adds weakref support for TypeVars. I haven't tried to debug the segfaults in the cloudpickle tests, but let me know if those can be minified to something that shows the issue is in CPython.

[3.12] gh-106403: Restore weakref support for TypeVar and friends (GH…

Closing this as the original report is (believed to be) fixed. If the other cloudpickle issues are caused by bugs in 3.12 it probably deserves its own issue.

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3.12 regression: cannot create weak reference to 'typing.TypeVar' object #106403

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3.12 regression: cannot create weak reference to 'typing.TypeVar' object #106403

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JelleZijlstra commented on Jul 4, 2023

ogrisel commented on Jul 6, 2023

JelleZijlstra commented on Jul 11, 2023

Yhg1s commented on Jul 31, 2023

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