I want to type hint a dictionary (or equivalent object) that maps from a set of Literal choices to some outputs. I'm using pytype as my type checker. The ideal behaviour would be something like: from typing import Literal SupportedKey = Literal["one", "two"] d : dict[SupportedKey, int] = {"one": 1, "two": 2} However, pytype doesn't recognise that "one" and "two" are in fact SupportedKeys, and so fails with Type annotation for d does not match type of assignment [annotation-type-mismatch] Annotation: dict[Literal['one', 'two'], int] Assignment: dict[str, Literal[1, 2]] Note that the mismatch in type assignment only matters in the keys, as: str is not assignable to Literal['one', 'two'] Literal[1, 2] is assignable to int I tried a more explicit method, where the keys are constructed with appropriate typing: from typing import Literal, Final SupportedKey = Literal["one", "two"] KEY_ONE: Final[SupportedKey] = "one" KEY_TWO: Final[SupportedKey] = "two" d : dict[SupportedKey, int] = {KEY_ONE: 1, KEY_TWO: 2} However, this fails in a similar way - the keys are still inferred as the more general str rather than Literal: Type annotation for d does not match type of assignment [annotation-type-mismatch] Annotation: dict[Literal['one', 'two'], int] Assignment: dict[str, int] What's the best way of achieving a correctly typed mapping with Literals as the keys? Continue reading...
