mutable.py 文件源码

def _parents(self):
        """Dictionary of parent object->attribute name on the parent.

        This attribute is a so-called "memoized" property.  It initializes
        itself with a new ``weakref.WeakKeyDictionary`` the first time
        it is accessed, returning the same object upon subsequent access.

        """

        return weakref.WeakKeyDictionary()