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The slice type doesn't participate in GC, which means that if you happen to create a cycle involving a slice, that cycle will never get freed. Here's an example:
def f():
 l = []
 l.append(slice(l))
# Will consume memory without bound:
while True:
 f()
This seems pretty hard to trigger accidentally, so it might not be a huge deal -- especially since it seems to have been around for a while. (I only checked 2.7 and trunk though.)
I think this could be solved by either having the slice class participate in GC (ie add tp_traverse and tp_clear methods), or maybe doing some type-filtering during slice_new().

Implementing tp_traverse & tp_clear seems to runs into complications due to slice_cache

New changeset 879da9400529 by Benjamin Peterson in branch '2.7':
add gc support to slice (closes #26659)
https://hg.python.org/cpython/rev/879da9400529
New changeset 9e2176d18965 by Benjamin Peterson in branch '3.5':
add gc support to slice (closes #26659)
https://hg.python.org/cpython/rev/9e2176d18965
New changeset 870fcc50f1bd by Benjamin Peterson in branch 'default':
merge 3.5 (#26659)
https://hg.python.org/cpython/rev/870fcc50f1bd