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<H3><A NAME="SECTION0082143000000000000000"></A><A NAME="optparse-raising-errors-in-callback"></A>
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6.21.4.3 Raising errors in a callback
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The callback function should raise OptionValueError if there are any
problems with the option or its argument(s).  <tt class="module">optparse</tt> 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.

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<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
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