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# 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[:])
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