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<title>6.21.2.8 How optparse handles errors
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<H3><A NAME=
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6.21.2.8 How
<tt class=
"module">optparse
</tt> handles errors
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.
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")
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.
Consider the first example above, where the user passes
<code>"4x"</code> to an
option that takes an integer:
<div class=
"verbatim"><pre>
foo: error: option -n: invalid integer value: '
4x'
Or, where the user fails to pass a value at all:
<div class=
"verbatim"><pre>
foo: error: -n option requires an argument
<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.
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>.
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<span class=
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