# The cycle GC collector can be executed when any GC-tracked object is

# allocated, e.g. during a call to PyList_New(), PyDict_New(), ...

# Moreover, it can invoke arbitrary Python code via a weakref callback.

# This means that there are many places in the source where an arbitrary

# mutation could unexpectedly occur.

# The example below shows list_slice() not expecting the call to

# PyList_New to mutate the input list.  (Of course there are many

# more examples like this one.)

import weakref

class A(object):

    pass

def callback(x):

    del lst[:]

keepalive = []

for i in range(100):

    lst = [str(i)]

    a = A()

    a.cycle = a

    keepalive.append(weakref.ref(a, callback))

    del a

    while lst:

        keepalive.append(lst[:])