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<div class="section" id="the-parse-args-method">
<h1>The parse_args() method<a class="headerlink" href="#the-parse-args-method" title="Permalink to this headline">¶</a></h1>
<dl class="method">
<dt id="parse_args">
<tt class="descname">parse_args</tt><big>(</big><span class="optional">[</span><em>args</em><span class="optional">]</span><span class="optional">[</span>, <em>namespace</em><span class="optional">]</span><big>)</big><a class="headerlink" href="#parse_args" title="Permalink to this definition">¶</a></dt>
<dd><p>Convert the strings to objects and assign them as attributes of the namespace. Return the populated namespace.</p>
<p>Previous calls to <a title="add_argument" class="reference external" href="add_argument.html#add_argument"><tt class="xref docutils literal"><span class="pre">add_argument()</span></tt></a> determine exactly what objects are created and how they are assigned. See the documentation for <a title="add_argument" class="reference external" href="add_argument.html#add_argument"><tt class="xref docutils literal"><span class="pre">add_argument()</span></tt></a> for details.</p>
<p>By default, the arg strings are taken from <tt class="docutils literal"><span class="pre">sys.argv</span></tt>, and a new empty <tt class="docutils literal"><span class="pre">Namespace</span></tt> object is created for the attributes.</p>
</dd></dl>
<div class="section" id="invalid-arguments">
<h2>Invalid arguments<a class="headerlink" href="#invalid-arguments" title="Permalink to this headline">¶</a></h2>
<p>While parsing the command-line, <tt class="docutils literal"><span class="pre">parse_args</span></tt> checks for a variety of errors, including invalid types, invalid options, wrong number of positional arguments, etc. When it encounters such an error, it exits and prints the error along with a usage message:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s">'PROG'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">'--foo'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">'bar'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s">'?'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="c"># invalid type</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s">'--foo'</span><span class="p">,</span> <span class="s">'spam'</span><span class="p">])</span>
<span class="go">usage: PROG [-h] [--foo FOO] [bar]</span>
<span class="go">PROG: error: argument --foo: invalid int value: 'spam'</span>
<span class="gp">>>> </span><span class="c"># invalid option</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s">'--bar'</span><span class="p">])</span>
<span class="go">usage: PROG [-h] [--foo FOO] [bar]</span>
<span class="go">PROG: error: no such option: --bar</span>
<span class="gp">>>> </span><span class="c"># wrong number of arguments</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s">'spam'</span><span class="p">,</span> <span class="s">'badger'</span><span class="p">])</span>
<span class="go">usage: PROG [-h] [--foo FOO] [bar]</span>
<span class="go">PROG: error: extra arguments found: badger</span>
</pre></div>
</div>
<p>The <tt class="docutils literal"><span class="pre">parse_args</span></tt> method attempts to give errors whenever the user has clearly made a mistake, but some situations are inherently ambiguous. For example, the command-line arg <tt class="docutils literal"><span class="pre">'-1'</span></tt> could either be an attempt to specify an option or an attempt to provide a positional argument. The <tt class="docutils literal"><span class="pre">parse_args</span></tt> method is cautious here: positional arguments may only begin with <tt class="docutils literal"><span class="pre">'-'</span></tt> if they look like negative numbers and there are no options in the parser that look like negative numbers:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s">'PROG'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">'-x'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">'foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s">'?'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="c"># no negative number options, so -1 is a positional argument</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s">'-x'</span><span class="p">,</span> <span class="s">'-1'</span><span class="p">])</span>
<span class="go">Namespace(foo=None, x='-1')</span>
<span class="gp">>>> </span><span class="c"># no negative number options, so -1 and -5 are positional arguments</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s">'-x'</span><span class="p">,</span> <span class="s">'-1'</span><span class="p">,</span> <span class="s">'-5'</span><span class="p">])</span>
<span class="go">Namespace(foo='-5', x='-1')</span>
<span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">(</span><span class="n">prog</span><span class="o">=</span><span class="s">'PROG'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">'-1'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s">'one'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">'foo'</span><span class="p">,</span> <span class="n">nargs</span><span class="o">=</span><span class="s">'?'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="c"># negative number options present, so -1 is an option</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s">'-1'</span><span class="p">,</span> <span class="s">'X'</span><span class="p">])</span>
<span class="go">Namespace(foo=None, one='X')</span>
<span class="gp">>>> </span><span class="c"># negative number options present, so -2 is an option</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s">'-2'</span><span class="p">])</span>
<span class="go">usage: PROG [-h] [-1 ONE] [foo]</span>
<span class="go">PROG: error: no such option: -2</span>
<span class="gp">>>> </span><span class="c"># negative number options present, so both -1s are options</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s">'-1'</span><span class="p">,</span> <span class="s">'-1'</span><span class="p">])</span>
<span class="go">usage: PROG [-h] [-1 ONE] [foo]</span>
<span class="go">PROG: error: argument -1: expected one argument</span>
</pre></div>
</div>
<p>If you have positional arguments that must begin with <tt class="docutils literal"><span class="pre">'-'</span></tt> and don’t look like negative numbers, you can insert the pseudo-argument <tt class="docutils literal"><span class="pre">'--'</span></tt> which tells <tt class="docutils literal"><span class="pre">parse_args</span></tt> that everything after that is a positional argument:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s">'--'</span><span class="p">,</span> <span class="s">'-f'</span><span class="p">])</span>
<span class="go">Namespace(foo='-f', one=None)</span>
</pre></div>
</div>
</div>
<div class="section" id="beyond-sys-argv">
<h2>Beyond <tt class="docutils literal"><span class="pre">sys.argv</span></tt><a class="headerlink" href="#beyond-sys-argv" title="Permalink to this headline">¶</a></h2>
<p>Sometimes it may be useful to have an ArgumentParser parse args other than those of <tt class="docutils literal"><span class="pre">sys.argv</span></tt>. This can be accomplished by passing a list of strings to <tt class="docutils literal"><span class="pre">parse_args</span></tt>. You may have noticed that the examples in the argparse documentation have made heavy use of this calling style - it is much easier to use at the interactive prompt:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span>
<span class="gp">... </span> <span class="s">'integers'</span><span class="p">,</span> <span class="n">metavar</span><span class="o">=</span><span class="s">'int'</span><span class="p">,</span> <span class="nb">type</span><span class="o">=</span><span class="nb">int</span><span class="p">,</span> <span class="n">choices</span><span class="o">=</span><span class="nb">xrange</span><span class="p">(</span><span class="mf">10</span><span class="p">),</span>
<span class="gp">... </span> <span class="n">nargs</span><span class="o">=</span><span class="s">'+'</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">'an integer in the range 0..9'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span>
<span class="gp">... </span> <span class="s">'--sum'</span><span class="p">,</span> <span class="n">dest</span><span class="o">=</span><span class="s">'accumulate'</span><span class="p">,</span> <span class="n">action</span><span class="o">=</span><span class="s">'store_const'</span><span class="p">,</span> <span class="n">const</span><span class="o">=</span><span class="nb">sum</span><span class="p">,</span>
<span class="gp">... </span> <span class="n">default</span><span class="o">=</span><span class="nb">max</span><span class="p">,</span> <span class="n">help</span><span class="o">=</span><span class="s">'sum the integers (default: find the max)'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">([</span><span class="s">'1'</span><span class="p">,</span> <span class="s">'2'</span><span class="p">,</span> <span class="s">'3'</span><span class="p">,</span> <span class="s">'4'</span><span class="p">])</span>
<span class="go">Namespace(accumulate=<built-in function max>, integers=[1, 2, 3, 4])</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="s">'1 2 3 4 --sum'</span><span class="o">.</span><span class="n">split</span><span class="p">())</span>
<span class="go">Namespace(accumulate=<built-in function sum>, integers=[1, 2, 3, 4])</span>
</pre></div>
</div>
</div>
<div class="section" id="custom-namespaces">
<h2>Custom namespaces<a class="headerlink" href="#custom-namespaces" title="Permalink to this headline">¶</a></h2>
<p>It may also be useful to have an ArgumentParser assign attributes to an already existing object, rather than the newly-created Namespace object that is normally used. This can be achieved by specifying the <tt class="docutils literal"><span class="pre">namespace=</span></tt> keyword argument:</p>
<div class="highlight-python"><div class="highlight"><pre><span class="gp">>>> </span><span class="k">class</span> <span class="nc">C</span><span class="p">(</span><span class="nb">object</span><span class="p">):</span>
<span class="gp">... </span> <span class="k">pass</span>
<span class="gp">...</span>
<span class="gp">>>> </span><span class="n">c</span> <span class="o">=</span> <span class="n">C</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">parser</span> <span class="o">=</span> <span class="n">argparse</span><span class="o">.</span><span class="n">ArgumentParser</span><span class="p">()</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">add_argument</span><span class="p">(</span><span class="s">'--foo'</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">parser</span><span class="o">.</span><span class="n">parse_args</span><span class="p">(</span><span class="n">args</span><span class="o">=</span><span class="p">[</span><span class="s">'--foo'</span><span class="p">,</span> <span class="s">'BAR'</span><span class="p">],</span> <span class="n">namespace</span><span class="o">=</span><span class="n">c</span><span class="p">)</span>
<span class="gp">>>> </span><span class="n">c</span><span class="o">.</span><span class="n">foo</span>
<span class="go">'BAR'</span>
</pre></div>
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<h3><a href="index.html">Table Of Contents</a></h3>
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<li><a class="reference external" href="">The parse_args() method</a><ul>
<li><a class="reference external" href="#invalid-arguments">Invalid arguments</a></li>
<li><a class="reference external" href="#beyond-sys-argv">Beyond <tt class="docutils literal"><span class="pre">sys.argv</span></tt></a></li>
<li><a class="reference external" href="#custom-namespaces">Custom namespaces</a></li>
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