test_argparse.py has some "choices='abc'" cases.  

In those should "parser.parse_args(['--foo','bc'])" be considered a success or failure?  

The underlying issue here is that while string iteration behaves like list iteration, string __contains__ looks for substrings, not just one character that matches.  (String __contains__ also returns a TypeError if its argument is not a string.)

But other than removing poor examples in documentation and tests, I'm not sure this issue requires a change.