WeakKeyIDDictionary simple implementation

import collections.abc as cabc
import weakref


class WeakKeyIDDictionary(cabc.MutableMapping):
    def __init__(self):  # TODO: Allow passing parameters here
        self._ids = weakref.WeakValueDictionary()  # from id(key) to key
        self._values = {}  # from id(key) to value

    def _remove_dead_refs(self):
        """Remove items from self._values which are not in self._ids.

        Return true if the dictionary changed.

        """
        strong_ids = list(self._ids.values())  # hold strong references to
                                               # keep the dictionary stable
        dead_identifiers = self._values.keys() - self._ids.keys()
        for identifier in dead_identifiers:
            del self._values[identifier]
        return bool(dead_identifiers)


    def __getattr__(self, key):
        # We have a strong reference to key, so it won't vanish.
        # Calling _remove_dead_refs() would not affect the correctness of this
        # method.
        identifier = id(key)
        return self._values[identifier]

    def __setattr__(self, key, value):
        # We have a strong reference to key, so again it won't vanish.
        identifier = id(key)
        self._ids[identifier] = key
        self._values[identifier] = value

    def __delattr__(self, key):
        # Strong reference, won't vanish.
        del self._ids[identifier]
        del self._values[identifier]

    def __iter__(self):
        for identifier, key in self._ids.items():
            # Have a strong ref to key
            # And the dictionary isn't allowed to change during iteration
            # anyway
            yield key

    def __len__(self):
        return len(self._ids)