[1.20 regression] _Environ.get(key, default) returns str | None when default is an alias to Any

Since mypy 1.20, passing a value whose type is an alias to Any (e.g. Incomplete: TypeAlias = Any) as the default argument of os.environ.get causes mypy to infer str | None instead of Any, producing a spurious [arg-type] error at the use site.

Reproducer: https://mypy-play.net/?gist=7a79f64ac9733542b571cc358f6d3165

import os
from pathlib import Path
from typing import Any
from typing_extensions import TypeAlias

Incomplete: TypeAlias = Any

A: Incomplete = Path()
reveal_type(A)               # Any  (good)

B = os.environ.get('TEST', A)
reveal_type(B)               # str | None  (bad)  (expected: Any)

C = Path(B)                  # error: Argument 1 to "Path" has incompatible
                             # type "str | None"; expected "str | PathLike[str]"

Bisected to 30260ca57 which added explicit _Environ.get/pop overloads where the first overload is def get(self, key, default: None = None) -> AnyStr | None. Before that sync, _Environ inherited Mapping.get, whose first overload took no default argument, so 2-arg calls couldn't match it.

Versions: broken on mypy ≥ 1.20.0; works on mypy ≤ 1.19.1.

Bisects to this typeshed sync #20505

Looks like it'll be python/typeshed#15194

So either typeshed needs to be changed, or the following behaviour of mypy needs to be changed:

from typing import Any, Generic, TypeVar, overload
from typing_extensions import TypeAlias

_T = TypeVar("_T")

class Env:
    @overload
    def get(self, key: str, default: None = None) -> str | None: ...
    @overload
    def get(self, key: str, default: str) -> str: ...
    @overload
    def get(self, key: str, default: _T) -> str | _T: ...

Incomplete: TypeAlias = Any

def main(unknown: Incomplete, env: Env):
    reveal_type(env.get('key', unknown))

This is similar to e.g. pyright

Reminds me of python/typeshed#13222

This is the reason i opened #21345 (sorry for not linking it here)

Navigation Menu

Repository navigation

[1.20 regression] _Environ.get(key, default) returns str | None when default is an alias to Any #21344

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

[1.20 regression] _Environ.get(key, default) returns str | None when default is an alias to Any #21344

Description

Activity

hauntsaninja commented on Apr 27, 2026

hauntsaninja commented on Apr 27, 2026

hauntsaninja commented on Apr 27, 2026

hauntsaninja commented on Apr 27, 2026

pablogsal commented on Apr 27, 2026

Metadata

Metadata

Assignees

Labels

Projects

Milestone

Relationships

Development

Issue actions