Add bound Hashable for types for keys of type Mapping and values of type Set

Problem

Types used for keys in mappings and values in sets are too broad.

How to reproduce

d: [list, int] = {}  # Tools that use typeshed see this code as valid.
d[[1]] = 2 # TypeError: unhashable type: 'list'

Solution

Change type of keys in mappings: _KT = TypeVar("_KT") to _KT = TypeVar("_KT", bound=Hashable)

• Added PR with fixes to stdlib

But I still have questions about how to fix the types in set and frozenset.

• For set, we using _T, which is also used for values in the list. Can I change it to _KT? The requirements for the type of values in the set are the same as for the keys in the dictionary. I understand that it is a value, not a key, and the name can be confusing. Do you think of a better option?

• For frozenset, we using covariant _T_co, which is also used for values in the tuple. It's the same problem here. We need a new type that is covariant and bound to Hashable.

• set

• frozenset

And once we figure out what to do with types in stdlib, I will be happy to add the same changes to third-party libraries.

Comienza revisando stdlib/builtins.pyi en las definiciones de set y frozenset referenciadas en la issue, junto con los cambios relacionados en los mappings de PR 9570. Decide cómo deben representarse la variable de tipo de set y una variable covariante de frozenset acotada por Hashable; después, actualiza los stubs de stdlib y verifica que las claves no hasheables o los valores set sean rechazados por las herramientas de comprobación de tipos relevantes.