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[:])