Create case-insensitive Literal type hint

[tvdboom]

Hi,

I am trying to create a Literal type hint that's case-insensitive, since I don't want to bother the user with case-sensitive nonsense.
I have created this class, that works...

import typing
from beartype.typing import Annotated
from beartype.vale import Is

class Literal(typing.Literal):
    def __class_getitem__(cls, X: tuple[str]) -> Annotated[str, Is]:
        return Annotated[str, Is[lambda x: x.lower() in X]]

... but mypy complains. For example, when having a function like this

def func(item: Literal["a", "b"] = "a"): ...

it raises this error: error: Incompatible default for argument "item" (default has type "str", argument has type "Literal"). So mypy doesn't recognize my class Literal as a str, while this works for the original typing.Literal.

Any idea how to fix this? Thanks

[leycec]

Fascinating... and exceedingly clever, too. That's just what we've all come to expect from @tvdboom.

Surprisingly, this now is beginning to seem like more of a mypy issue rather than a @beartype issue. I blame them! Mypy is bad and should feel bad.

Specifically, mypy fails to extend the same semantics applied to standard typing classes to subclasses of those standard typing classes. If mypy accepts a function annotated by typing.Literal[A, B] for any valid literals A and B, then mypy should also accept the same function annotated by YourLiteral[A, B] for any subclass class YourLiteral(typing.Literal):.

But... it doesn't. So, that's a mypy bug. Right? I think? I hope? Please let this not be @beartype's responsibility. Odin, hear our cry! 👻

[tvdboom]

Fair enough. Looking at the issue tracker of mypy, I think my changes are slim of getting any help there... I'll try to find a workaround while I wait for an answer (python/mypy#16172). Thanks anyway

[leycec]

...yeah. Pretty much. Static type-checkers really have no incentive to resolve issues specific to runtime type-checkers like @beartype. We are the competition. They'd prefer us to just go away.

The only workaround I can think of is the obvious (and very, very dumb) workaround: litter your code with filthy # type: ignore[{whatevah}] comments every time you want to use your homebrew Literal type. Of course, this is yet another reason why static type-checkers are bad for everyone's health: these inscrutable, highly opinionated, and mostly non-configurable black boxes are wrong about almost everything.

"Mostly non-configurable." That's the problem. Sure, you could try to configure mypy through mypy-specific plugins. That would probably work. But pylance, which is even more popular than mypy, fails to provide a similar plugin API. There's currently no standard general-purpose off-the solution to customize static type-checking behaviour.

Thus, I sigh. 😮‍💨