Investigate `mypy` warning `alt.theme.register`

[dangotbanned]

What happened?

Causing an upstream warning in https://github.com/narwhals-dev/narwhals/actions/runs/13154290048/job/36707803255

altair/theme.py:251: error: ParamSpec "P" is unbound  [valid-type]
            def wrapper(*args: P.args, **kwargs: P.kwargs) -> ThemeConfig:
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Found 1 error in 1 file (checked 383 source files)

altair/altair/theme.py

Lines 250 to 252 in 7fce53d

	@_wraps(func)
	def wrapper(*args: P.args, **kwargs: P.kwargs) -> ThemeConfig:
	return func(*args, **kwargs)

Seems like mypy has suddenly decided it doesn't know about

altair/altair/theme.py

Line 99 in 7fce53d

P = ParamSpec("P")

What would you like to happen instead?

Not trigger a failure in narwhals

Which version of Altair are you using?

7fce53d

[dangotbanned]

Maybe resolvable by replacing alt.theme.P with:

altair/altair/utils/plugin_registry.py

Lines 24 to 25 in 7fce53d

	R = TypeVar("R")
	Plugin = TypeAliasType("Plugin", Callable[..., R], type_params=(R,))

Edit 1

Or changing Plugin to this, playing with PEP 696 and importing alt.utils.plugin_registry.P:

P = ParamSpec("P")
R = TypeVar("R")
Plugin = TypeAliasType("Plugin", Callable[P, R], type_params=(P, R))

Edit 2

python/mypy#18278 (comment)

Hello hello! There's nothing that puts P into scope / nothing that tells a type checker what P is meant to match.

If you changed def decorate(func: Plugin[ThemeConfig], /) -> Plugin[ThemeConfig]: to def decorate(func: Callable[P, ThemeConfig], /) -> Callable[P, ThemeConfig]: that would do the job and actually provide better type checking to callers. (Looks like Plugin is basically just an alias for Callable that loses track of parameter types)

It's also unclear to me why you need an inner function at all. Seems like you could just return func from decorate and not bother with wrapper. This would also be a fix.