:class:`~unittest.TestSuite` now drops references to tests as soon as the test

has been run, if the test is successful. On Python interpreters that do

garbage collection, this allows the tests to be garbage collected if nothing

else is holding a reference to the test. It is possible to override this

behavior by creating a :class:`~unittest.TestSuite` subclass that defines a

custom ``_removeTestAtIndex`` method. (Contributed by Tom Wardill, Matt

McClure, and Andrew Svetlov in :issue:`11798`.)

* Because :mod:`unittest.TestSuite` now drops references to tests after they

are run, test harnesses that re-use a :class:`~unittest.TestSuite` to re-run

a set of tests may fail. Test suites should not be re-used in this fashion

since it means state is retained between test runs, breaking the test

isolation that :mod:`unittest` is designed to provide. However, if the lack

of isolation is considered acceptable, the old behavior can be restored by

creating a :mod:`~unittest.TestSuite` subclass that defines a

``_removeTestAtIndex`` method that does nothing (see

:meth:`.TestSuite.__iter__`) (:issue:`11798`).