test_gc.py 文件源码

def test_boom(self):
        class Boom:
            def __getattr__(self, someattribute):
                del self.attr
                raise AttributeError

        a = Boom()
        b = Boom()
        a.attr = b
        b.attr = a

        gc.collect()
        garbagelen = len(gc.garbage)
        del a, b
        # a<->b are in a trash cycle now.  Collection will invoke
        # Boom.__getattr__ (to see whether a and b have __del__ methods), and
        # __getattr__ deletes the internal "attr" attributes as a side effect.
        # That causes the trash cycle to get reclaimed via refcounts falling to
        # 0, thus mutating the trash graph as a side effect of merely asking
        # whether __del__ exists.  This used to (before 2.3b1) crash Python.
        # Now __getattr__ isn't called.
        self.assertEqual(gc.collect(), 4)
        self.assertEqual(len(gc.garbage), garbagelen)