[ Python 常見問題 ] Type hint for a function that returns only a specific set of values

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Question
I have a function that can only return a, b or c, all of them are of type T. And 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

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

Is that the correct one? I know that I can do this

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1. def fun(...) -> T:  
2.     #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.

HowTo
You can do that with literal types.

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1. from typing_extensions import Literal  
2. # from typing import Literal  # Python 3.8 or higher  
3.   
4. def fun(b: int) -> Literal["a", "b", "c"]:  
5.     if b == 0:  
6.         return "a"  
7.     if b == 1:  
8.         return "b"  
9.     return "d"  

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

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1. error: Incompatible return value type (got "Literal['d']",  
2. 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.