Python TypeMap; a container holding at most one value of a given type

typemap.py

from typing import Type
from typing import TypeVar


class ValueTypeError(ValueError):
    """Raised when a `TypeMap` receives a key-value pair where ``type(value) != key``."""

    def __init__(self, key: type, value: object, message: str) -> None:
        """Construct a `ValueTypeError`.

        :param key: The key received by a `TypeMap`
        :param value: The value received by a `TypeMap`
        :param message: A developer-facing description of what went wrong
        """
        super().__init__(message)
        self.key = key
        self.value = value
        self.message = message


T = TypeVar("T")


class TypeMap(dict[type, object]):
    """A heterogeneous container holding at most one value of a given type.

    A `TypeMap` can hold any number of values, but not more than one for a given
    type; it can hold one `int`, one `str`, one `dict`, one `bool`, and so on.
    
    .. seealso:: https://docs.rs/anymap/
    """

    def __getitem__(self, k: Type[T]) -> T:
        return self[k]

    def __setitem__(self, k: Type[T], v: T) -> None:
        if k != type(v):
            raise ValueTypeError(
                k,
                v,
                f"Attempted to set {k!r} = {v!r}, but value has type {type(v)!r}, not {k!r}",
            )
        self[k] = v