Proposal: Allow using final literal immutable containers as Literal

[antonagestam]

I've found many situations where it would have been preferable to be able to use a Literal at runtime, or to be able to dynamically check if a value is in it. I'm proposing that a frozenset (and perhaps other immutable containers) should be interpreted as a Literal if defined as a literal, used as a type hint, and perhaps required to be defined with ClassVar or Final. This would allow sharing definitions of valid choices between runtime and static type checking.

Further I'm proposing to add type-guards for ... in ... statements using these Literal-like collections.

An example of how I'm thinking this should work:

AllowedMethod: Final = frozenset({"POST", "GET", "PUT", "PATCH", "HEAD"})


# This should work as if defined as method: Literal["POST", "GET", "PUT", "PATCH", "HEAD"]
def request(method: AllowedMethod) -> Response:
    ...


def do_thing(method: str) -> Response:
    # Testing for membership should narrow `method` to `AllowedMethod`
    if method in AllowedMethod:
        request(method)

[antonagestam]

I'm closing this as I've found solving this using phantom types to work well. Here's an example using ISO country codes if anyone stumbles upon this issue: https://github.com/antonagestam/phantom-types/blob/main/phantom/ext/iso3166.py