Diff from mypy_primer, showing the effect of this PR on open source code:

sphinx (https://github.com/sphinx-doc/sphinx.git)
- sphinx/util/logging.py: note: In member "handle" of class "SphinxLoggerAdapter":
- sphinx/util/logging.py:140:9: error: Item "LoggerAdapter" of "Union[Logger, LoggerAdapter]" has no attribute "handle"

porcupine (https://github.com/Akuli/porcupine.git)
+ porcupine/plugins/langserver.py:129: error: Missing type parameters for generic type "LoggerAdapter"  [type-arg]
+ porcupine/plugins/langserver.py:143: error: Missing type parameters for generic type "LoggerAdapter"  [type-arg]
+ porcupine/plugins/langserver.py:323: error: Missing type parameters for generic type "LoggerAdapter"  [type-arg]
+ porcupine/plugins/langserver.py:724: error: Missing type parameters for generic type "LoggerAdapter"  [type-arg]

Cc @Akuli as someone affected by this change.

@Akuli @srittau is it an issue that one has to distinct between type checking and runtime now? I mean, LoggerAdapter is not subscriptable, so the type has to be either quoted

x: "logging.LoggerAdapter[logging.Logger]"  = ...

or aliased via e.g.

if TYPE_CHECKING:
    import logging
    LoggerAdapter = logging.LoggerAdapter[logging.Logger]
else:
    from logging import LoggerAdapter

x: LoggerAdapter = ...

That's a general issue, but temporary. I would recommend to use from __future__ import annotations if possible or quoting of not possible.

@srittau @hoefling What would be the recommendation for inheritance? From what I understand importing annotations or quoting would not be as useful here

import logging

class Adapter(logging.LoggerAdapter[logging.Logger]):
    pass

As LoggerAdapter is not subscriptable, this class will fail to initialise

This is a common problem, and unfortunately it's annoying to deal with. This works, but it's messy and feels like a hack:

import logging
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    _LoggerAdapter = logging.LoggerAdapter[logging.Logger]
else:
    _LoggerAdapter = logging.LoggerAdapter

class MyAdapter(_LoggerAdapter):
    pass

Things that would help, but don't exist yet:

• Default values for generics (Defaults for Generics? mypy#4236)
• Pull request to CPython that makes logging.LoggerAdapter generic at runtime (similar PRs have been accepted in the past)

Linking the relevant section of mypy docs, in case it helps people: https://mypy.readthedocs.io/en/stable/runtime_troubles.html#using-classes-that-are-generic-in-stubs-but-not-at-runtime

I've opened python/cpython#92129 to add __class_getitem__ to the class in 3.11+.

(Update: merged!)