Thanks for the input, r.david.murray.  I've updated my patch and attached it to take into consideration your comments: test_argparse.py.COLUMNS.update2.patch.  The updated patch uses EnviormentVarGuard as suggested, except that it slightly tweaks EnviormentVarGuard so the context manager protocol methods don't have to be invoked directly.

It was also pointed out that "adding setUp and tearDown to TestCase isn't enough, since subclasses and mixins define those without calling the superclass versions", which is true.  However, the tests that override setUp() happen to be those that don't depend on the COLUMNS environment variable.