sam-t2/devtools/amd64/html/python/lib/optparse-default-values.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-generating-help.html" />
<link rel="prev" href="optparse-other-actions.html" />
<link rel="parent" href="optparse-tutorial.html" />
<link rel="next" href="optparse-generating-help.html" />
<meta name='aesop' content='information' />
<title>6.21.2.5 Default values</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="6.21.2.4 Other actions"
  href="optparse-other-actions.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="6.21.2.6 Generating help"
  href="optparse-generating-help.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-other-actions.html">6.21.2.4 Other actions</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-generating-help.html">6.21.2.6 Generating help</A>
</div>
<hr /></div>
</DIV>
<!--End of Navigation Panel-->

<H3><A NAME="SECTION0082125000000000000000"></A><A NAME="optparse-default-values"></A>
<BR>
6.21.2.5 Default values
</H3>

<P>
All of the above examples involve setting some variable (the
``destination'') when certain command-line options are seen.  What happens
if those options are never seen?  Since we didn't supply any defaults,
they are all set to <code>None</code>.  This is usually fine, but sometimes you
want more control.  <tt class="module">optparse</tt> lets you supply a default value for each
destination, which is assigned before the command line is parsed.

<P>
First, consider the verbose/quiet example.  If we want <tt class="module">optparse</tt> to set
<code>verbose</code> to <code>True</code> unless <code>"-q"</code> is seen, then we can do this:
<div class="verbatim"><pre>
parser.add_option("-v", action="store_true", dest="verbose", default=True)
parser.add_option("-q", action="store_false", dest="verbose")
</pre></div>

<P>
Since default values apply to the <em>destination</em> rather than to any
particular option, and these two options happen to have the same
destination, this is exactly equivalent:
<div class="verbatim"><pre>
parser.add_option("-v", action="store_true", dest="verbose")
parser.add_option("-q", action="store_false", dest="verbose", default=True)
</pre></div>

<P>
Consider this:
<div class="verbatim"><pre>
parser.add_option("-v", action="store_true", dest="verbose", default=False)
parser.add_option("-q", action="store_false", dest="verbose", default=True)
</pre></div>

<P>
Again, the default value for <code>verbose</code> will be <code>True</code>: the last
default value supplied for any particular destination is the one that
counts.

<P>
A clearer way to specify default values is the <tt class="method">set_defaults()</tt>
method of OptionParser, which you can call at any time before calling
<tt class="method">parse_args()</tt>:
<div class="verbatim"><pre>
parser.set_defaults(verbose=True)
parser.add_option(...)
(options, args) = parser.parse_args()
</pre></div>

<P>
As before, the last value specified for a given option destination is
the one that counts.  For clarity, try to use one method or the other of
setting default values, not both.

<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="6.21.2.4 Other actions"
  href="optparse-other-actions.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="6.21.2.6 Generating help"
  href="optparse-generating-help.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-other-actions.html">6.21.2.4 Other actions</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-generating-help.html">6.21.2.6 Generating help</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>