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<div class="section" id="default-values">
<h1>Default values<a class="headerlink" href="#default-values" title="Permalink to this headline"></a></h1>
<p>For elements such as arguments and options, if <tt class="docutils literal"><span class="pre">default</span></tt> property is set, its value is stored in case the element was not used.</p>
<div class="highlight-lua"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4
5</pre></div></td><td class="code"><div class="highlight"><pre><span class="n">parser</span><span class="p">:</span><span class="n">option</span><span class="p">(</span><span class="s2">&quot;</span><span class="s">-o --output&quot;</span><span class="p">,</span> <span class="s2">&quot;</span><span class="s">Output file.&quot;</span><span class="p">,</span> <span class="s2">&quot;</span><span class="s">a.out&quot;</span><span class="p">)</span>
<span class="c1">-- Equivalent:</span>
<span class="n">parser</span><span class="p">:</span><span class="n">option</span> <span class="s2">&quot;</span><span class="s">-o&quot;</span> <span class="s2">&quot;</span><span class="s">--output&quot;</span>
<span class="p">:</span><span class="n">description</span> <span class="s2">&quot;</span><span class="s">Output file.&quot;</span>
<span class="p">:</span><span class="n">default</span> <span class="s2">&quot;</span><span class="s">a.out&quot;</span>
</pre></div>
</td></tr></table></div>
<div class="highlight-python"><div class="highlight"><pre>$ lua script.lua
</pre></div>
</div>
<div class="highlight-lua"><div class="highlight"><pre><span class="p">{</span>
<span class="n">output</span> <span class="o">=</span> <span class="s2">&quot;</span><span class="s">a.out&quot;</span>
<span class="p">}</span>
</pre></div>
</div>
<p>The existence of a default value is reflected in help message, unless <tt class="docutils literal"><span class="pre">show_default</span></tt> property is set to <tt class="docutils literal"><span class="pre">false</span></tt>.</p>
<div class="highlight-python"><div class="highlight"><pre>$ lua script.lua --help
</pre></div>
</div>
<div class="highlight-python"><div class="highlight"><pre>Usage: script.lua [-o &lt;output&gt;] [-h]
Options:
-o &lt;output&gt;, --output &lt;output&gt;
Output file. (default: a.out)
-h, --help Show this help message and exit.
</pre></div>
</div>
<p>Note that invocation without required arguments is still an error.</p>
<div class="highlight-python"><div class="highlight"><pre>$ lua script.lua -o
</pre></div>
</div>
<div class="highlight-python"><div class="highlight"><pre>Usage: script.lua [-o &lt;output&gt;] [-h]
Error: too few arguments
</pre></div>
</div>
<div class="section" id="default-mode">
<h2>Default mode<a class="headerlink" href="#default-mode" title="Permalink to this headline"></a></h2>
<p><tt class="docutils literal"><span class="pre">defmode</span></tt> property regulates how argparse should use the default value of an element.</p>
<p>If <tt class="docutils literal"><span class="pre">defmode</span></tt> contains <tt class="docutils literal"><span class="pre">u</span></tt> (for unused), the default value will be automatically passed to the element if it was not invoked at all. This is the default behavior.</p>
<p>If <tt class="docutils literal"><span class="pre">defmode</span></tt> contains <tt class="docutils literal"><span class="pre">a</span></tt> (for argument), the default value will be automatically passed to the element if not enough arguments were passed, or not enough invocations were made.</p>
<p>Consider the difference:</p>
<div class="highlight-lua"><table class="highlighttable"><tr><td class="linenos"><div class="linenodiv"><pre>1
2
3
4
5</pre></div></td><td class="code"><div class="highlight"><pre><span class="n">parser</span><span class="p">:</span><span class="n">option</span> <span class="s2">&quot;</span><span class="s">-o&quot;</span>
<span class="p">:</span><span class="n">default</span> <span class="s2">&quot;</span><span class="s">a.out&quot;</span>
<span class="n">parser</span><span class="p">:</span><span class="n">option</span> <span class="s2">&quot;</span><span class="s">-p&quot;</span>
<span class="p">:</span><span class="n">default</span> <span class="s2">&quot;</span><span class="s">password&quot;</span>
<span class="p">:</span><span class="n">defmode</span> <span class="s2">&quot;</span><span class="s">arg&quot;</span>
</pre></div>
</td></tr></table></div>
<div class="highlight-python"><div class="highlight"><pre>$ lua script.lua -h
</pre></div>
</div>
<div class="highlight-python"><div class="highlight"><pre>Usage: script.lua [-o &lt;o&gt;] [-p [&lt;p&gt;]] [-h]
Options:
-o &lt;o&gt; default: a.out
-p [&lt;p&gt;] default: password
-h, --help Show this help message and exit.
</pre></div>
</div>
<div class="highlight-python"><div class="highlight"><pre>$ lua script.lua
</pre></div>
</div>
<div class="highlight-lua"><div class="highlight"><pre><span class="p">{</span>
<span class="n">o</span> <span class="o">=</span> <span class="s2">&quot;</span><span class="s">a.out&quot;</span>
<span class="p">}</span>
</pre></div>
</div>
<div class="highlight-python"><div class="highlight"><pre>$ lua script.lua -p
</pre></div>
</div>
<div class="highlight-lua"><div class="highlight"><pre><span class="p">{</span>
<span class="n">o</span> <span class="o">=</span> <span class="s2">&quot;</span><span class="s">a.out&quot;</span><span class="p">,</span>
<span class="n">p</span> <span class="o">=</span> <span class="s2">&quot;</span><span class="s">password&quot;</span>
<span class="p">}</span>
</pre></div>
</div>
<div class="highlight-python"><div class="highlight"><pre>$ lua script.lua -o
</pre></div>
</div>
<div class="highlight-python"><div class="highlight"><pre>Usage: script.lua [-o &lt;o&gt;] [-p [&lt;p&gt;]] [-h]
Error: too few arguments
</pre></div>
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