1 2 # The cycle GC collector can be executed when any GC-tracked object is 3 # allocated, e.g. during a call to PyList_New(), PyDict_New(), ... 4 # Moreover, it can invoke arbitrary Python code via a weakref callback. 5 # This means that there are many places in the source where an arbitrary 6 # mutation could unexpectedly occur. 7 8 # The example below shows list_slice() not expecting the call to 9 # PyList_New to mutate the input list. (Of course there are many 10 # more examples like this one.) 11 12 13 import weakref 14 15 class A(object): 16 pass 17 18 def callback(x): 19 del lst[:] 20 21 22 keepalive = [] 23 24 for i in range(100): 25 lst = [str(i)] 26 a = A() 27 a.cycle = a 28 keepalive.append(weakref.ref(a, callback)) 29 del a 30 while lst: 31 keepalive.append(lst[:]) 32
