Shared via mypy Playground

main.py

import typing
import types
from typing import Any


def _validate_type_hint(parameter_type: type[Any]) -> tuple[str, Any] | None:
    generic_args: tuple[type[Any], ...] = typing.get_args(parameter_type)

    match generic_args:
        case () | (_,):
            # This is just misuse of this function. A Union type cannot have less than two type args.
            raise ValueError(f'UnionType cannot have only {len(generic_args)} type argument(s).')
        case (_, types.NoneType) | (types.NoneType, _):
            # Proper path
            pass
        case (first, second):
            return 'Multi-type parameter must have only one non-None type', f'"{first.__name__}" and "{second.__name__}""'
        case _:
            return 'Must be either "<type>", "Sequence[<type>], or <type> | None', f'"{parameter_type}" is not valid'
    return None