import weakref

class C:
    __slots__ = ['x', 'y', '__weakref__']
    def __init__(self):
        self.x = None
    def setY(self):
        if self.x.x:
            self.y = D(weakref.ref(self.x.x))

class D:
    def __init__(self, wr):
        self.wr = wr
    def __del__(self):
        o = self.wr()

root = c = C()
for n in range(100):
    c.x = c = C()

c = root
while c.x:
    c.setY()
    c = c.x

print('deleting')
del root
print('done')