Class WeakKeyDictionary

Lib/weakref.py:298–439

Mapping class that references keys weakly. Entries in the dictionary will be discarded when there is no longer a strong reference to the key. This can be used to associate additional data with an object owned by other parts of an application without adding attributes to those objec

class WeakKeyDictionary(_collections_abc.MutableMapping):
    """ Mapping class that references keys weakly.

    Entries in the dictionary will be discarded when there is no
    longer a strong reference to the key. This can be used to
    associate additional data with an object owned by other parts of
    an application without adding attributes to those objects. This
    can be especially useful with objects that override attribute
    accesses.
    """

    def __init__(self, dict=None):
        self.data = {}
        def remove(k, selfref=ref(self)):
            self = selfref()
            if self is not None:
                try:
                    del self.data[k]
                except KeyError:
                    pass
        self._remove = remove
        if dict is not None:
            self.update(dict)

    def __delitem__(self, key):
        del self.data[ref(key)]

    def __getitem__(self, key):
        return self.data[ref(key)]

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

    def __repr__(self):
        return "<%s at %#x>" % (self.__class__.__name__, id(self))

    def __setitem__(self, key, value):
        self.data[ref(key, self._remove)] = value

    def copy(self):
        new = WeakKeyDictionary()
        for key, value in self.data.copy().items():
            o = key()
            if o is not None:
                new[o] = value
        return new

    __copy__ = copy

    def __deepcopy__(self, memo):
        from copy import deepcopy
        new = self.__class__()
        for key, value in self.data.copy().items():
            o = key()
            if o is not None:
                new[o] = deepcopy(value, memo)
        return new