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<h2 class="title"><a name="apihelper.optional"></a>4.2. Using Optional and Named Arguments
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<div class="abstract">
<p><span class="application">Python</span> allows function arguments to have default values; if the function is called without the argument, the argument gets its default
value. Futhermore, arguments can be specified in any order by using named arguments. Stored procedures in <span class="application">SQL Server</span> Transact/<span class="acronym">SQL</span> can do this, so if you're a <span class="application">SQL Server</span> scripting guru, you can skim this part.
</p>
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<div class="informalexample">
<p>Here is an example of <tt class="function">info</tt>, a function with two optional arguments:
</p><pre class="programlisting"><span class='pykeyword'>
def</span> info(object, spacing=10, collapse=1):</pre></div>
<p><tt class="varname">spacing</tt> and <tt class="varname">collapse</tt> are optional, because they have default values defined. <tt class="varname">object</tt> is required, because it has no default value. If <tt class="function">info</tt> is called with only one argument, <tt class="varname">spacing</tt> defaults to <tt class="constant">10</tt> and <tt class="varname">collapse</tt> defaults to <tt class="constant">1</tt>. If <tt class="function">info</tt> is called with two arguments, <tt class="varname">collapse</tt> still defaults to <tt class="constant">1</tt>.
</p>
<p>Say you want to specify a value for <tt class="varname">collapse</tt> but want to accept the default value for <tt class="varname">spacing</tt>. In most languages, you would be out of luck, because you would need to call the function with three arguments. But in
<span class="application">Python</span>, arguments can be specified by name, in any order.
</p>
<div class="example"><a name="d0e8401"></a><h3 class="title">Example 4.4. Valid Calls of <tt class="function">info</tt></h3><pre class="programlisting">
info(odbchelper) <a name="apihelper_args.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
info(odbchelper, 12) <a name="apihelper_args.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
info(odbchelper, collapse=0) <a name="apihelper_args.1.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
info(spacing=15, object=odbchelper) <a name="apihelper_args.1.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12"></pre><div class="calloutlist">
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<td valign="top" align="left">With only one argument, <tt class="varname">spacing</tt> gets its default value of <tt class="literal">10</tt> and <tt class="varname">collapse</tt> gets its default value of <tt class="constant">1</tt>.
</td>
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<td width="12" valign="top" align="left"><a href="#apihelper_args.1.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
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<td valign="top" align="left">With two arguments, <tt class="varname">collapse</tt> gets its default value of <tt class="constant">1</tt>.
</td>
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<td valign="top" align="left">Here you are naming the <tt class="varname">collapse</tt> argument explicitly and specifying its value. <tt class="varname">spacing</tt> still gets its default value of <tt class="literal">10</tt>.
</td>
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<td valign="top" align="left">Even required arguments (like <tt class="varname">object</tt>, which has no default value) can be named, and named arguments can appear in any order.
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<p>This looks totally whacked until you realize that arguments are simply a dictionary. The “<span class="quote">normal</span>” method of calling functions without argument names is actually just a shorthand where <span class="application">Python</span> matches up the values with the argument names in the order they're specified in the function declaration. And most of the
time, you'll call functions the “<span class="quote">normal</span>” way, but you always have the additional flexibility if you need it.
</p><a name="tip.arguments"></a><table class="note" border="0" summary="">
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<td colspan="2" align="left" valign="top" width="99%">The only thing you need to do to call a function is specify a value (somehow) for each required argument; the manner and order
in which you do that is up to you.
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<div class="furtherreading">
<h3>Further Reading on Optional Arguments</h3>
<ul>
<li><a href="http://www.python.org/doc/current/tut/tut.html"><i class="citetitle"><span class="application">Python</span> Tutorial</i></a> discusses exactly <a href="http://www.python.org/doc/current/tut/node6.html#SECTION006710000000000000000">when and how default arguments are evaluated</a>, which matters when the default value is a list or an expression with side effects.
</li>
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