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      1 """
      2 The gc module can still invoke arbitrary Python code and crash.
      3 This is an attack against _PyInstance_Lookup(), which is documented
      4 as follows:
      5 
      6     The point of this routine is that it never calls arbitrary Python
      7     code, so is always "safe":  all it does is dict lookups.
      8 
      9 But of course dict lookups can call arbitrary Python code.
     10 The following code causes mutation of the object graph during
     11 the call to has_finalizer() in gcmodule.c, and that might
     12 segfault.
     13 """
     14 
     15 import gc
     16 
     17 
     18 class A:
     19     def __hash__(self):
     20         return hash("__del__")
     21     def __eq__(self, other):
     22         del self.other
     23         return False
     24 
     25 a = A()
     26 b = A()
     27 
     28 a.__dict__[b] = 'A'
     29 
     30 a.other = b
     31 b.other = a
     32 
     33 gc.collect()
     34 del a, b
     35 
     36 gc.collect()
     37