Type hint for a list of possible values [duplicate]

Question

I have a function which can take a fixed list of values: e.g.

def func(mode="a"):
   if mode not in ["a", "b"]:
      raise AttributeError("not ok")

is there a way to type hint it can only be one of these two values?

Answer 1

I think you want a literal type:

def func(mode: Literal["a", "b"] = "a"):
    if mode not in ["a", "b"]:
        raise AttributeError("not ok")

This was introduced in Python 3.8, via PEP 586.