Covariant `Mapping`s key type?

[LeeeeT]

Describe the problem

Why is Mappings key type made covariant? This is already discussed and showed to be unsafe in python/typing#273. This contradicts type theory.

Gist to reproduce

No response

Severity

annoying but i can live with it

Your Environment

No response

[KotlinIsland]

see #62

it's covariant because it's safe. the only case in which it would be unsafe is when someone writes a __getitem__/get, disables the error that mypy produces, and then additionally misuses the input value in an unsafe way:

playground

thanks for looking into basedmypy, if you have any questions i'm happy to discuss. we also have a discord if your interested

[LeeeeT]

Do I understand this correctly? Every time someone tries to write a __getitem__ / get mypy will produce an error (more like a warning) and if they are sure the usage is safe they can ignore the error in this case, right?

[KotlinIsland]

yes, that's correct. although it is most definitely an error (albeit with a lengthy note), we don't emit warnings yet

not entirely correct though, the author is entirely free to write a get that does not use any type parameters and opt for object if they wish. you won't receive an error in this case

[LeeeeT]

Thanks for the explanation ❤️