Python MultiDict, in a copy-paste-able library, because I find myself needing them surprisingly often

multidict.py

from collections.abc import ItemsView, Set, KeysView
from typing import Iterable, Iterator, NamedTuple, Hashable

class MultiDict[K: Hashable, V: Hashable]:
    """
    Associates sets of values with a key.
    Values can repeat for different keys if needed.
    """
    def __init__(self):
        self.__vals_by_key: dict[K, set[V]] = {}

    def add(self, key: K, value: V):
        self.__vals_by_key.setdefault(key, set())
        self.__vals_by_key[key].add(value)

    def has_single(self, key: K, value: V) -> bool:
        value_set = self.__vals_by_key.get(key, set())
        return value in value_set

    def has_key(self, key: K) -> bool:
        value_set = self.__vals_by_key.get(key)
        if value_set is None:
            return False
        assert value_set, "there must not be empty sets"
        return True

    def remove_single(self, key: K, value: V):
        value_set = self.__vals_by_key[key]
        value_set.remove(value)
        if not value_set:
            del self.__vals_by_key[key]

    def remove_key(self, key: K):
        del self.__vals_by_key[key]

    def discard_single(self, key: K, value: V):
        value_set = self.__vals_by_key.get(key)
        if not value_set:
            return
        value_set.discard(value)
        if not value_set:
            del self.__vals_by_key[key]
    
    def discard_key(self, key: K):
        self.__vals_by_key.pop(key, None)

    def clear(self):
        self.__vals_by_key.clear()

    def keys(self) -> KeysView[K]:
        return self.__vals_by_key.keys()
    
    def values(self) -> Iterator[V]:
        for value_set in self.__vals_by_key.values():
            assert value_set, "there must not be empty sets"
            yield from value_set
    
    def items(self) -> ItemsView[K, Set[V]]:
        return self.__vals_by_key.items()
    
    def get(self, key: K) -> Set[V]:
        return self.__vals_by_key.get(key, set())
    
    def __getitem__(self, key: K) -> Set[V]:
        return self.__vals_by_key[key]
    
    def count_keys(self) -> int:
        return len(self.__vals_by_key)
    
    def __contains__(self, key: K) -> bool:
        return self.has_key(key)
    
    def __len__(self) -> int:
        return self.count_keys()

    def __iter__(self) -> Iterator[K]:
        return iter(self.keys())