How can I type hint a dictionary where the key is a specific tuple and the value is known?

Question

How can I type hint a dictionary where the key is a specific tuple and the value is known?

For example I want to type hint a dict like this:

class A:
    pass

class B:
    pass

class_map: = {
    (str,): A
    (int,): B
}

some_cls = class_map[(str,)]

The use case will be to go from a known set of bases to a class that was previously defined using those bases.

Answer 1

One can do this by

• making a new class ClassMap which will allow dict key lookup
  • note: dict cannot be subclassed because our __getitem__ signature differs from the one that dict uses
• in ClassMap define __getitem__ which gets values from an input dict
• in ClassMap define an overload of __getitem__ with the tuple input and type hint output
• making an instance of the ClassMap
• use it and type hints work
• passes mypy check

Another thing that we can do is require that the input to make the dict is a frozenset of tuples. Then one can type hint what is allowed in:

tuple_items: frozenset[
    typing.Union[
        typing.Tuple[typing_extensions.Literal['1invalid'], int],
        typing.Tuple[typing_extensions.LiteralString, float]
    ]
] = frozenset({
    ('1invalid', 1),
    ('a', 1.234)
})

This enables functionality similar to TypedDict but with tuple keys.

import typing

class A:
    pass


class B:
    pass


class ClassMap:
    def __init__(self, data: dict):
        self.__data = data

    @typing.overload
    def __getitem__(self, name: typing.Tuple[typing.Type[str]]) -> typing.Type[A]: ...

    @typing.overload
    def __getitem__(self, name: typing.Tuple[typing.Type[int]]) -> typing.Type[B]: ...

    def __getitem__(self, name):
        return self.__data[name]

class_map = ClassMap({
    (str,): A
    (int,): B
})


some_cls = class_map[(str,)]  # sees A, works!