Using a pipe symbol in typing.Literal string

Question

I have a function that accepts certain literals for a specific argument:

from typing import Literal

def fn(x: Literal["foo", "bar", "foo|bar"]) -> None:
    reveal_type(x)

The third contains a pipe symbol (|), "foo|bar". This is interpreted by mypy as an error, as the name foo is not defined.

I guess this happens due to how forward references are evaluated? I use Python 3.8 with:

from __future__ import annotations

Is there a way to make this work? I can not change the string due to breaking backward compatibility, but currently, the whole annotation is revealed as Any, i.e. it holds no value.

It would be helpful, too, if in your minimal reproducible example you showed the mypy usage you're using to demonstrate that "the whole annotation is revealed as Any" — Charles Duffy
– Charles Duffy, Commented Oct 29, 2023 at 21:02
As far as I can tell, this is a bug in mypy. Other type checkers (pyright, PyCharm inspections) can handle this without issue. I don't see an existing issue for it, so it would be worth reporting — Brian61354270
– Brian61354270, Commented Oct 29, 2023 at 21:20
Going by how, by design, Literal['Color.RED'] interprets 'Color.RED' as a string instead of a forward reference, I think mypy should be changed to interpret "foo|bar" as a string instead of a forward reference here too. — user2357112
– user2357112, Commented Oct 29, 2023 at 21:21

InSync · Accepted Answer · 2024-04-23 14:43:16Z

2

This bug is now fixed. The change hasn't been released yet, however.

answered Apr 23, 2024 at 14:43

InSync

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