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<h2 class="title"><a name="apihelper.getattr"></a>4.4. Getting Object References With <tt class="function">getattr</tt></h2>
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<div></div>
</div>
<div class="toc">
<ul>
<li><span class="section"><a href="getattr.html#d0e9194">4.4.1. getattr with Modules</a></span></li>
<li><span class="section"><a href="getattr.html#d0e9362">4.4.2. getattr As a Dispatcher</a></span></li>
</ul>
</div>
<div class="abstract">
<p>You already know that <a href="../getting_to_know_python/everything_is_an_object.html" title="2.4. Everything Is an Object"><span class="application">Python</span> functions are objects</a>. What you don't know is that you can get a reference to a function without knowing its name until run-time, by using the
<tt class="function">getattr</tt> function.
</p>
</div>
<div class="example"><a name="apihelper.getattr.intro"></a><h3 class="title">Example 4.10. Introducing <tt class="function">getattr</tt></h3><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput">li = [<span class='pystring'>"Larry"</span>, <span class='pystring'>"Curly"</span>]</span>
<tt class="prompt">>>> </tt><span class="userinput">li.pop</span> <a name="apihelper.getattr.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput"><built-in method pop of list object at 010DF884></span>
<tt class="prompt">>>> </tt><span class="userinput">getattr(li, <span class='pystring'>"pop"</span>)</span> <a name="apihelper.getattr.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput"><built-in method pop of list object at 010DF884></span>
<tt class="prompt">>>> </tt><span class="userinput">getattr(li, <span class='pystring'>"append"</span>)(<span class='pystring'>"Moe"</span>)</span> <a name="apihelper.getattr.1.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput">li</span>
<span class="computeroutput">["Larry", "Curly", "Moe"]</span>
<tt class="prompt">>>> </tt><span class="userinput">getattr({}, <span class='pystring'>"clear"</span>)</span> <a name="apihelper.getattr.1.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12">
<span class="computeroutput"><built-in method clear of dictionary object at 00F113D4></span>
<tt class="prompt">>>> </tt><span class="userinput">getattr((), <span class='pystring'>"pop"</span>)</span> <a name="apihelper.getattr.1.5"></a><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12">
<span class="traceback">Traceback (innermost last):
File "<interactive input>", line 1, in ?
AttributeError: 'tuple' object has no attribute 'pop'</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.1.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">This gets a reference to the <tt class="function">pop</tt> method of the list. Note that this is not calling the <tt class="function">pop</tt> method; that would be <tt class="literal">li.pop()</tt>. This is the method itself.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.1.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">This also returns a reference to the <tt class="function">pop</tt> method, but this time, the method name is specified as a string argument to the <tt class="function">getattr</tt> function. <tt class="function">getattr</tt> is an incredibly useful built-in function that returns any attribute of any object. In this case, the object is a list,
and the attribute is the <tt class="function">pop</tt> method.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.1.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">In case it hasn't sunk in just how incredibly useful this is, try this: the return value of <tt class="function">getattr</tt> <span class="emphasis"><em>is</em></span> the method, which you can then call just as if you had said <tt class="literal">li.append("Moe")</tt> directly. But you didn't call the function directly; you specified the function name as a string instead.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.1.4"><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="function">getattr</tt> also works on dictionaries.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.1.5"><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">In theory, <tt class="function">getattr</tt> would work on tuples, except that <a href="../native_data_types/tuples.html#odbchelper.tuplemethods" title="Example 3.16. Tuples Have No Methods">tuples have no methods</a>, so <tt class="function">getattr</tt> will raise an exception no matter what attribute name you give.
</td>
</tr>
</table>
</div>
</div>
<div class="section" lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a name="d0e9194"></a>4.4.1. <tt class="function">getattr</tt> with Modules
</h3>
</div>
</div>
<div></div>
</div>
<p><tt class="function">getattr</tt> isn't just for built-in datatypes. It also works on modules.
</p>
<div class="example"><a name="apihelper.getattr.example"></a><h3 class="title">Example 4.11. The <tt class="function">getattr</tt> Function in <tt class="filename">apihelper.py</tt></h3><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>import</span> odbchelper</span>
<tt class="prompt">>>> </tt><span class="userinput">odbchelper.buildConnectionString</span> <a name="apihelper.getattr.2.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput"><function buildConnectionString at 00D18DD4></span>
<tt class="prompt">>>> </tt><span class="userinput">getattr(odbchelper, <span class='pystring'>"buildConnectionString"</span>)</span> <a name="apihelper.getattr.2.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput"><function buildConnectionString at 00D18DD4></span>
<tt class="prompt">>>> </tt><span class="userinput">object = odbchelper</span>
<tt class="prompt">>>> </tt><span class="userinput">method = <span class='pystring'>"buildConnectionString"</span></span>
<tt class="prompt">>>> </tt><span class="userinput">getattr(object, method)</span> <a name="apihelper.getattr.2.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<span class="computeroutput"><function buildConnectionString at 00D18DD4></span>
<tt class="prompt">>>> </tt><span class="userinput">type(getattr(object, method))</span> <a name="apihelper.getattr.2.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12">
<span class="computeroutput"><type 'function'></span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>import</span> types</span>
<tt class="prompt">>>> </tt><span class="userinput">type(getattr(object, method)) == types.FunctionType</span>
<span class="computeroutput">True</span>
<tt class="prompt">>>> </tt><span class="userinput">callable(getattr(object, method))</span> <a name="apihelper.getattr.2.5"></a><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12">
<span class="computeroutput">True</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.2.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">This returns a reference to the <tt class="function">buildConnectionString</tt> function in the <tt class="filename">odbchelper</tt> module, which you studied in <a href="../getting_to_know_python/index.html" title="Chapter 2. Your First Python Program">Chapter 2, <i>Your First Python Program</i></a>. (The hex address you see is specific to my machine; your output will be different.)
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.2.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Using <tt class="function">getattr</tt>, you can get the same reference to the same function. In general, <tt class="literal"><tt class="function">getattr</tt>(<i class="replaceable">object</i>, "<i class="replaceable">attribute</i>")</tt> is equivalent to <tt class="literal"><i class="replaceable">object</i>.<i class="replaceable">attribute</i></tt>. If <i class="replaceable"><tt>object</tt></i> is a module, then <i class="replaceable"><tt>attribute</tt></i> can be anything defined in the module: a function, class, or global variable.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.2.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">And this is what you actually use in the <tt class="function">info</tt> function. <tt class="varname">object</tt> is passed into the function as an argument; <tt class="varname">method</tt> is a string which is the name of a method or function.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.2.4"><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">In this case, <tt class="varname">method</tt> is the name of a function, which you can prove by getting its <a href="built_in_functions.html#apihelper.type.intro" title="Example 4.5. Introducing type"><tt class="function">type</tt></a>.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.2.5"><img src="../images/callouts/5.png" alt="5" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Since <tt class="varname">method</tt> is a function, it is <a href="built_in_functions.html#apihelper.builtin.callable" title="Example 4.8. Introducing callable">callable</a>.
</td>
</tr>
</table>
</div>
</div>
</div>
<div class="section" lang="en">
<div class="titlepage">
<div>
<div>
<h3 class="title"><a name="d0e9362"></a>4.4.2. <tt class="function">getattr</tt> As a Dispatcher
</h3>
</div>
</div>
<div></div>
</div>
<p>A common usage pattern of <tt class="function">getattr</tt> is as a dispatcher. For example, if you had a program that could output data in a variety of different formats, you could
define separate functions for each output format and use a single dispatch function to call the right one.
</p>
<p>For example, let's imagine a program that prints site statistics in <span class="acronym">HTML</span>, <span class="acronym">XML</span>, and plain text formats. The choice of output format could be specified on the command line, or stored in a configuration
file. A <tt class="filename">statsout</tt> module defines three functions, <tt class="function">output_html</tt>, <tt class="function">output_xml</tt>, and <tt class="function">output_text</tt>. Then the main program defines a single output function, like this:
</p>
<div class="example"><a name="apihelper.getattr.dispatch"></a><h3 class="title">Example 4.12. Creating a Dispatcher with <tt class="function">getattr</tt></h3><pre class="programlisting"><span class='pykeyword'>
import</span> statsout
<span class='pykeyword'>def</span><span class='pyclass'> output</span>(data, format=<span class='pystring'>"text"</span>): <a name="apihelper.getattr.3.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
output_function = getattr(statsout, <span class='pystring'>"output_%s"</span> % format) <a name="apihelper.getattr.3.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class='pykeyword'>return</span> output_function(data) <a name="apihelper.getattr.3.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
</pre></div>
<div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.3.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The <tt class="function">output</tt> function takes one required argument, <tt class="varname">data</tt>, and one optional argument, <tt class="varname">format</tt>. If <tt class="varname">format</tt> is not specified, it defaults to <tt class="literal">text</tt>, and you will end up calling the plain text output function.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.3.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">You concatenate the <tt class="varname">format</tt> argument with "output_" to produce a function name, and then go get that function from the <tt class="filename">statsout</tt> module. This allows you to easily extend the program later to support other output formats, without changing this dispatch
function. Just add another function to <tt class="filename">statsout</tt> named, for instance, <tt class="function">output_pdf</tt>, and pass "pdf" as the <tt class="varname">format</tt> into the <tt class="function">output</tt> function.
</td>
</tr>
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.3.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Now you can simply call the output function in the same way as any other function. The <tt class="varname">output_function</tt> variable is a reference to the appropriate function from the <tt class="filename">statsout</tt> module.
</td>
</tr>
</table>
</div>
<p>Did you see the bug in the previous example? This is a very loose coupling of strings and functions, and there is no error
checking. What happens if the user passes in a format that doesn't have a corresponding function defined in <tt class="filename">statsout</tt>? Well, <tt class="function">getattr</tt> will return <tt class="literal">None</tt>, which will be assigned to <tt class="varname">output_function</tt> instead of a valid function, and the next line that attempts to call that function will crash and raise an exception. That's
bad.
</p>
<p>Luckily, <tt class="function">getattr</tt> takes an optional third argument, a default value.
</p>
<div class="example"><a name="apihelper.getattr.default"></a><h3 class="title">Example 4.13. <tt class="function">getattr</tt> Default Values
</h3><pre class="programlisting"><span class='pykeyword'>
import</span> statsout
<span class='pykeyword'>def</span><span class='pyclass'> output</span>(data, format=<span class='pystring'>"text"</span>):
output_function = getattr(statsout, <span class='pystring'>"output_%s"</span> % format, statsout.output_text)
<span class='pykeyword'>return</span> output_function(data) <a name="apihelper.getattr.4.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
</pre><div class="calloutlist">
<table border="0" summary="Callout list">
<tr>
<td width="12" valign="top" align="left"><a href="#apihelper.getattr.4.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">This function call is guaranteed to work, because you added a third argument to the call to <tt class="function">getattr</tt>. The third argument is a default value that is returned if the attribute or method specified by the second argument wasn't
found.
</td>
</tr>
</table>
</div>
</div>
<p>As you can see, <tt class="function">getattr</tt> is quite powerful. It is the heart of introspection, and you'll see even more powerful examples of it in later chapters.
</p>
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