The ''_expand_help' method formats one action (argument) at a time, so it could issue an error message that includes that action's name.

The disconnect that you noticed arises because your bad 'help' parameter wasn't tested until is was used in a 'print_help'.

http://bugs.python.org/issue9849 asks for better testing of `nargs` (and `metavar`) values.  In the proposed patch, 'add_argument' creates a temporary HelpFormatter and tries to format a relevant portion of the help.  I suppose that test could be extended to test the 'help' parameter as well.