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<title>6.21.2.5 Default values
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<H3><A NAME=
"SECTION0082125000000000000000"></A><A NAME=
"optparse-default-values"></A>
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.
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")
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)
<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)
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
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)
(options, args) = parser.parse_args()
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.
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<span class=
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2.4.2, documentation updated on
28 September
2005.
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