os.environ.get() accepts `default` as keyword argument, but ty reports no-matching-overload

[adamtheturtle]

Summary

import os

os.environ.get("FOO", default="BAR")

This code works but ty errors.
mypy and pyright pass.

Version

ty 0.0.8 (aa7559d 2025-12-29)

[adamtheturtle]

Perhaps related: python/cpython#124675 (comment) - dict is not strictly a Mapping.

[MichaReiser]

I suspect this is #1714, given that Mapping is a Protocol.

[AlexWaygood]

I suspect this is #1714, given that Mapping is a Protocol.

No, I don't think #1714 is related here. Mapping is not a protocol, and ty doesn't think it is either:

from typing import Mapping, is_protocol, reveal_type

reveal_type(is_protocol(Mapping))  # Literal[False]

Mapping is an ABC, rather than a protocol. And even if it were a protocol, #1714 only concerns types that implicitly subtype protocols -- os._Environ explicitly inherits from MutableMapping[AnyStr, AnyStr] in typeshed, and MutableMapping in turn explicitly inherits from Mapping.

I'm not sure exactly what the issue is here, but I suspect it's some strangeness to do with constrained TypeVars.

[RasmusNygren]

I think this is just typeshed being incorrect. Mypy also fails (https://mypy-play.net/?mypy=master&python=3.12&gist=49d939f7441874364a23bb2e937d4032) if you run it towards the master branch that has the latest typeshed stubs.

python/typeshed#15085 changed the default parameter ofMapping.get to be positional-only and did not make any changes to os._Environ to accept default as a keyword.

Edit: opened up a typeshed issue for it: python/typeshed#15193

[carljm]

Will leave this open so we remember to validate the fix when an updated typeshed arrives, but it seems like the fix is only in typeshed. Thanks @RasmusNygren!

[AlexWaygood]

I confirmed that this was fixed by astral-sh/ruff#22321.

Thanks again for the report @adamtheturtle, and thanks for the typeshed fix @RasmusNygren!

[adamtheturtle]

Thanks folks for digging and fixing!

I hit this because I (built and) use mypy-strict-kwargs - a mypy plugin to enforce using keyword arguments where possible. That's one use of the mypy plugin system that I don't think is ever going to be possible with public plans for ty (which I understand to be "no plugins, but support the ecosystem so that plugins aren't necessary").