sam-t2/devtools/v9/html/python/lib/optparse-how-optparse-handles-errors.html

<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html>
<head>
<link rel="STYLESHEET" href="lib.css" type='text/css' />
<link rel="SHORTCUT ICON" href="../icons/pyfav.png" type="image/png" />
<link rel='start' href='../index.html' title='Python Documentation Index' />
<link rel="first" href="lib.html" title='Python Library Reference' />
<link rel='contents' href='contents.html' title="Contents" />
<link rel='index' href='genindex.html' title='Index' />
<link rel='last' href='about.html' title='About this document...' />
<link rel='help' href='about.html' title='About this document...' />
<link rel="next" href="optparse-putting-it-all-together.html" />
<link rel="prev" href="optparse-printing-version-string.html" />
<link rel="parent" href="optparse-tutorial.html" />
<link rel="next" href="optparse-putting-it-all-together.html" />
<meta name='aesop' content='information' />
<title>6.21.2.8 How optparse handles errors</title>
</head>
<body>
<DIV CLASS="navigation">
<div id='top-navigation-panel' xml:id='top-navigation-panel'>
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="6.21.2.7 Printing a version"
  href="optparse-printing-version-string.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="6.21.2 Tutorial"
  href="optparse-tutorial.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="6.21.2.9 Putting it all"
  href="optparse-putting-it-all-together.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="optparse-printing-version-string.html">6.21.2.7 Printing a version</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="optparse-tutorial.html">6.21.2 Tutorial</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="optparse-putting-it-all-together.html">6.21.2.9 Putting it all</A>
</div>
<hr /></div>
</DIV>
<!--End of Navigation Panel-->

<H3><A NAME="SECTION0082128000000000000000"></A><A NAME="optparse-how-optparse-handles-errors"></A>
<BR>
6.21.2.8 How <tt class="module">optparse</tt> handles errors
</H3>

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

<P>
Handling user errors is much more important, since they are guaranteed
to happen no matter how stable your code is.  <tt class="module">optparse</tt> can automatically
detect some user errors, such as bad option arguments (passing <code>"-n
4x"</code> where <b class="programopt">-n</b> takes an integer argument), missing arguments
(<code>"-n"</code> at the end of the command line, where <b class="programopt">-n</b> takes an argument
of any type).  Also, you can call <code>parser.error()</code> to signal an
application-defined error condition:
<div class="verbatim"><pre>
(options, args) = parser.parse_args()
[...]
if options.a and options.b:
    parser.error("options -a and -b are mutually exclusive")
</pre></div>

<P>
In either case, <tt class="module">optparse</tt> 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.

<P>
Consider the first example above, where the user passes <code>"4x"</code> to an
option that takes an integer:
<div class="verbatim"><pre>
$ /usr/bin/foo -n 4x
usage: foo [options]

foo: error: option -n: invalid integer value: '4x'
</pre></div>

<P>
Or, where the user fails to pass a value at all:
<div class="verbatim"><pre>
$ /usr/bin/foo -n
usage: foo [options]

foo: error: -n option requires an argument
</pre></div>

<P>
<tt class="module">optparse</tt>-generated error messages take care always to mention the option
involved in the error; be sure to do the same when calling
<code>parser.error()</code> from your application code.

<P>
If <tt class="module">optparse</tt>'s default error-handling behaviour does not suite your needs,
you'll need to subclass OptionParser and override <code>exit()</code> and/or
<tt class="method">error()</tt>.

<P>

<DIV CLASS="navigation">
<div class='online-navigation'>
<p></p><hr />
<table align="center" width="100%" cellpadding="0" cellspacing="2">
<tr>
<td class='online-navigation'><a rel="prev" title="6.21.2.7 Printing a version"
  href="optparse-printing-version-string.html"><img src='../icons/previous.png'
  border='0' height='32'  alt='Previous Page' width='32' /></A></td>
<td class='online-navigation'><a rel="parent" title="6.21.2 Tutorial"
  href="optparse-tutorial.html"><img src='../icons/up.png'
  border='0' height='32'  alt='Up One Level' width='32' /></A></td>
<td class='online-navigation'><a rel="next" title="6.21.2.9 Putting it all"
  href="optparse-putting-it-all-together.html"><img src='../icons/next.png'
  border='0' height='32'  alt='Next Page' width='32' /></A></td>
<td align="center" width="100%">Python Library Reference</td>
<td class='online-navigation'><a rel="contents" title="Table of Contents"
  href="contents.html"><img src='../icons/contents.png'
  border='0' height='32'  alt='Contents' width='32' /></A></td>
<td class='online-navigation'><a href="modindex.html" title="Module Index"><img src='../icons/modules.png'
  border='0' height='32'  alt='Module Index' width='32' /></a></td>
<td class='online-navigation'><a rel="index" title="Index"
  href="genindex.html"><img src='../icons/index.png'
  border='0' height='32'  alt='Index' width='32' /></A></td>
</tr></table>
<div class='online-navigation'>
<b class="navlabel">Previous:</b>
<a class="sectref" rel="prev" href="optparse-printing-version-string.html">6.21.2.7 Printing a version</A>
<b class="navlabel">Up:</b>
<a class="sectref" rel="parent" href="optparse-tutorial.html">6.21.2 Tutorial</A>
<b class="navlabel">Next:</b>
<a class="sectref" rel="next" href="optparse-putting-it-all-together.html">6.21.2.9 Putting it all</A>
</div>
</div>
<hr />
<span class="release-info">Release 2.4.2, documentation updated on 28 September 2005.</span>
</DIV>
<!--End of Navigation Panel-->
<ADDRESS>
See <i><a href="about.html">About this document...</a></i> for information on suggesting changes.
</ADDRESS>
</BODY>
</HTML>