import gc
gc.set_debug(gc.DEBUG_UNCOLLECTABLE|gc.DEBUG_COLLECTABLE|gc.DEBUG_OBJECTS)

class Cycle:
    def __init__(self):
        self.c = self

class Immortal:
    resurrected = []
    
    def __del__(self):
        self.resurrected.append(self)

class Finalizer:
    def __del__(self):
        pass

cycle = Cycle()
cycle.x = Immortal()
cycle.x.y = Finalizer()

print (gc.collect(), gc.garbage)
assert gc.garbage == []
# Nothing special so far

del cycle

print (gc.collect(), gc.garbage)
# 'Finalizer' instance appears in garbage.
# Not real garbage though:
del gc.garbage[0]
gc.collect()
assert gc.garbage == []