import gc
import xx
import weakref

class CF(list):
    __slots__ = []

    def __del__(self):
        print('running __del__ for CF', self)

    def callback(self, wr):
        # if this runs, it can do bad things: cause crashes, revive 'self'
        print('oops, should not run', repr(self))



class A:
    def __del__(self):
        print('running __del__ for A')



ct = xx.new() # does not implement tp_traverse
ct.a = A()

cf = CF()
cf.append(ct)
cf.append(cf) # create cycle

# second trash cycle containing weakref, created so it outlives cf cycle
wr = [weakref.ref(ct.a, cf.callback)]
wr.append(wr)

del ct, A
gc.collect()
del cf, wr

# At this point the following is true:
#   - cf and the weakref are cyclic trash
#   - ct is not yet trash (it actually is but the GC doesn't know because of
#     the missing tp_traverse method).  It will be freed with cf is cleared
#   - callback for 'a' will run when cf is cleared because ct drops the ref to
#     'a'

gc.set_debug(gc.DEBUG_COLLECTABLE)
print('running collect')
print(gc.collect())
gc.set_debug(0)