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6.21.2.8 How `optparse` handles errors

There are two broad classes of errors that `optparse` has to worry about: programmer errors and user errors. Programmer errors are usually erroneous calls to `parser.add_option()`, e.g. invalid option strings, unknown option attributes, missing option attributes, etc. These are dealt with in the usual way: raise an exception (either `optparse.OptionError` or `TypeError`) and let the program crash.

Handling user errors is much more important, since they are guaranteed to happen no matter how stable your code is. `optparse` can automatically detect some user errors, such as bad option arguments (passing `"-n 4x"` where -n takes an integer argument), missing arguments (`"-n"` at the end of the command line, where -n takes an argument of any type). Also, you can call `parser.error()` to signal an application-defined error condition:

    (options, args) = parser.parse_args()
    [...]
    if options.a and options.b:
        parser.error("options -a and -b are mutually exclusive")

In either case, `optparse` handles the error the same way: it prints the program's usage message and an error message to standard error and exits with error status 2.

Consider the first example above, where the user passes `"4x"` to an option that takes an integer:

    $ /usr/bin/foo -n 4x
    usage: foo [options]

    foo: error: option -n: invalid integer value: '4x'

Or, where the user fails to pass a value at all:

    $ /usr/bin/foo -n
    usage: foo [options]

    foo: error: -n option requires an argument

`optparse`-generated error messages take care always to mention the option involved in the error; be sure to do the same when calling `parser.error()` from your application code.

If `optparse`'s default error-handling behaviour does not suite your needs, you'll need to subclass OptionParser and override `exit()` and/or `error()`.