I have a function that can only return a, b or c, all of them are of type T. I want to include this fact in the signature because of the special meaning they carry in the context of the function. How do I do that?

Currently, I use this

def fun(...) -> "a or b or c":
    #briefly explain the meaning of a, b and c in its docstring

Is that the correct one?

I know that I can do this

def fun(...) -> T:
    # briefly explain the meaning of a, b and c in its docstring

but as I said, I want to express in the signature that the function only returns those specific values.

You can do that with literal types.

from typing_extensions import Literal
# from typing import Literal  # Python 3.8 or higher

def fun(b: int) -> Literal["a", "b", "c"]:
    if b == 0:
        return "a"
    if b == 1:
        return "b"
    return "d"

mypy is able to detect the return "d" as a invalid statement:

error: Incompatible return value type (got "Literal['d']",
expected "Union[Literal['a'], Literal['b'], Literal['c']]")

Python 3.8

Thanks to the PEP 586, the Literal is already included by default in the Python 3.8 typing module.