Converting str to Literal["a", "b", ...

[ghost]

Let's say we have a function that takes multiple Literal arguments.

def takes_a_literal(foo: Literal["a", "b"]) -> None:
    print(foo)

And we call that function, from a function that takes a string.

def does_something(foo: str) -> None:
    takes_a_literal(foo)

Obviously, this will fail because foo can be anything. But what if we want to be assertive?

assert foo in ["a", "b], reveal_type(foo) is still str.

I can't even cast it: cast(Literal["a", "b"], foo) -- reveal_type(foo) still says str.

The only thing I've found that works is this:

literal_foo: Literal["a", "b"]
if foo == "a":
    literal_foo = "a"
elif foo == "b":
    literal_foo = "b"

Then passing literal_foo. This gets quite wordy with more than a couple of Literals.

Is there another way to do this?

[JelleZijlstra]

This is a duplicate of #10977.