from typing import ChainMap, MutableMapping, TypeVar, cast

_KT = TypeVar("_KT")
_VT = TypeVar("_VT")


class DeepChainMap(ChainMap[_KT, _VT]):
    """Variant of ChainMap that allows direct updates to inner scopes.

    Only works when all passed mapping are mutable.
    """

    def __setitem__(self, key: _KT, value: _VT) -> None:
        for mapping in self.maps:
            mutable_mapping = cast(MutableMapping[_KT, _VT], mapping)
            if key in mutable_mapping:
                mutable_mapping[key] = value
                return
        cast(MutableMapping[_KT, _VT], self.maps[0])[key] = value

    def __delitem__(self, key: _KT) -> None:
        """
        Raises
        ------
        KeyError
            If `key` doesn't exist.
        """
        for mapping in self.maps:
            mutable_mapping = cast(MutableMapping[_KT, _VT], mapping)
            if key in mapping:
                del mutable_mapping[key]
                return
        raise KeyError(key)
