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<H3><A NAME="SECTION0082149000000000000000"></A><A NAME="optparse-callback-example-6"></A>
<BR>
6.21.4.9 Callback example 6: variable arguments
</H3>
<P>
Things get hairy when you want an option to take a variable number of
arguments. For this case, you must write a callback, as <tt class="module">optparse</tt> doesn't
provide any built-in capabilities for it. And you have to deal with
certain intricacies of conventional <span class="Unix">Unix</span> command-line parsing that <tt class="module">optparse</tt>
normally handles for you. In particular, callbacks should implement
the conventional rules for bare <code>"-"</code> and <code>"-"</code> arguments:
<UL>
<LI>
either <code>"-"</code> or <code>"-"</code> can be option arguments
<P>
</LI>
<LI>
bare <code>"-"</code> (if not the argument to some option): halt command-line
processing and discard the <code>"-"</code>
<P>
</LI>
<LI>
bare <code>"-"</code> (if not the argument to some option): halt command-line
processing but keep the <code>"-"</code> (append it to <code>parser.largs</code>)
<P>
</LI>
</UL>
<P>
If you want an option that takes a variable number of arguments, there
are several subtle, tricky issues to worry about. The exact
implementation you choose will be based on which trade-offs you're
willing to make for your application (which is why <tt class="module">optparse</tt> doesn't support
this sort of thing directly).
<P>
Nevertheless, here's a stab at a callback for an option with variable
arguments:
<div class="verbatim"><pre>
def vararg_callback(option, opt_str, value, parser):
assert value is None
done = 0
value = []
rargs = parser.rargs
while rargs:
arg = rargs[0]
# Stop if we hit an arg like "--foo", "-a", "-fx", "--file=f",
# etc. Note that this also stops on "-3" or "-3.0", so if
# your option takes numeric values, you will need to handle
# this.
if ((arg[:2] == "--" and len(arg) > 2) or
(arg[:1] == "-" and len(arg) > 1 and arg[1] != "-")):
break
else:
value.append(arg)
del rargs[0]
setattr(parser.values, option.dest, value)
[...]
parser.add_option("-c", "--callback",
action="callback", callback=varargs)
</pre></div>
<P>
The main weakness with this particular implementation is that negative
numbers in the arguments following <code>"-c"</code> will be interpreted as
further options (probably causing an error), rather than as arguments to
<code>"-c"</code>. Fixing this is left as an exercise for the reader.
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<span class="release-info">Release 2.4.3, documentation updated on 29 March 2006.</span>
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