DefaultEventLoopPolicy is not valid as a type / base class

[henryiii]

The following code fails mypy tmp.py:

import asyncio

class MyEventLoopPolicy(asyncio.DefaultEventLoopPolicy):
    pass

with:

tmp.py:3: error: Variable "asyncio.DefaultEventLoopPolicy" is not valid as a type
tmp.py:3: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
tmp.py:3: error: Invalid base class "asyncio.DefaultEventLoopPolicy"
Found 2 errors in 1 file (checked 1 source file)

This MWE is copied from the Python docs here: https://docs.python.org/3/library/asyncio-policy.html#custom-policies.

Maybe this is because of the assignment here:

typeshed/stdlib/asyncio/unix_events.pyi

Lines 94 to 99 in fd750bf

	class _UnixDefaultEventLoopPolicy(BaseDefaultEventLoopPolicy):
	def get_child_watcher(self) -> AbstractChildWatcher: ...
	def set_child_watcher(self, watcher: AbstractChildWatcher | None) -> None: ...
	SelectorEventLoop = _UnixSelectorEventLoop
	DefaultEventLoopPolicy = _UnixDefaultEventLoopPolicy

?

[AlexWaygood]

The bug appears to already be fixed on typeshed master as a result of #7343 (merged just seven days ago) 😃

mypy's bundled version of the typeshed stubs won't be updated until at least 0.940. For now, though, you can clone typeshed and use mypy with the --custom-typeshed-dir option, or by setting the MYPYPATH environment variable.

[srittau]

@AlexWaygood was faster, but I agree with the analysis.

[henryiii]

That's fantastic. While I don't fully understand why this fixed the problem, I can verify that does in fact now pass.

$ mypy tmp.py --custom-typeshed-dir ~/git/software/typeshed
Success: no issues found in 1 source file

[srittau]

Before, this line was in asyncio/__init__.pyi:

typeshed/stdlib/asyncio/__init__.pyi

Line 113 in 35bddd2

DefaultEventLoopPolicy: type[AbstractEventLoopPolicy]

This has been changed to a star import.

[AlexWaygood]

Yes, now the DefaultEventLoopPolicy name is imported either from asyncio.unix_events or asyncio.windows_events, depending on whether the machine running mypy is a Windows or Unix machine. This mirrors the behavior at runtime when importing DefaultEventLoopPolicy from asyncio.

[henryiii]

Ah, I missed that, thanks!