6.20.4.3 Error handling
The callback function should raise OptionValueError if
there are any problems with the option or its
argument(s). optparse catches this and terminates the
program, printing the error message you supply to stderr. Your
message should be clear, concise, accurate, and mention the option at
fault. Otherwise, the user will have a hard time figuring out what he
did wrong.
Release 2.3.3, documentation updated on December 19, 2003.
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