django1/django/dispatch/weakref_backports.py

"""
weakref_backports is a partial backport of the weakref module for python
versions below 3.4.

Copyright (C) 2013 Python Software Foundation, see license.python.txt for
details.

The following changes were made to the original sources during backporting:

 * Added `self` to `super` calls.
 * Removed `from None` when raising exceptions.

"""
from weakref import ref


class WeakMethod(ref):
    """
    A custom `weakref.ref` subclass which simulates a weak reference to
    a bound method, working around the lifetime problem of bound methods.
    """

    __slots__ = "_func_ref", "_meth_type", "_alive", "__weakref__"

    def __new__(cls, meth, callback=None):
        try:
            obj = meth.__self__
            func = meth.__func__
        except AttributeError:
            raise TypeError("argument should be a bound method, not {}"
                            .format(type(meth)))
        def _cb(arg):
            # The self-weakref trick is needed to avoid creating a reference
            # cycle.
            self = self_wr()
            if self._alive:
                self._alive = False
                if callback is not None:
                    callback(self)
        self = ref.__new__(cls, obj, _cb)
        self._func_ref = ref(func, _cb)
        self._meth_type = type(meth)
        self._alive = True
        self_wr = ref(self)
        return self

    def __call__(self):
        obj = super(WeakMethod, self).__call__()
        func = self._func_ref()
        if obj is None or func is None:
            return None
        return self._meth_type(func, obj)

    def __eq__(self, other):
        if isinstance(other, WeakMethod):
            if not self._alive or not other._alive:
                return self is other
            return ref.__eq__(self, other) and self._func_ref == other._func_ref
        return False

    def __ne__(self, other):
        if isinstance(other, WeakMethod):
            if not self._alive or not other._alive:
                return self is not other
            return ref.__ne__(self, other) or self._func_ref != other._func_ref
        return True

    __hash__ = ref.__hash__