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<title>6.4. Using sys.modules</title>
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<h2 class="title"><a name="fileinfo.modules"></a>6.4. Using <tt class="literal"><tt class="filename">sys</tt>.modules</tt></h2>
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<div></div>
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<div class="abstract">
<p>Modules, like everything else in <span class="application">Python</span>, are objects. Once imported, you can always get a reference to a module through the global dictionary <tt class="literal"><tt class="filename">sys</tt>.modules</tt>.
</p>
</div>
<div class="example"><a name="d0e15693"></a><h3 class="title">Example 6.12. Introducing <tt class="literal"><tt class="filename">sys</tt>.modules</tt></h3><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>import</span> sys</span> <a name="fileinfo.modules.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> <span class='pystring'>'\n'</span>.join(sys.modules.keys())</span> <a name="fileinfo.modules.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">win32api
os.path
os
exceptions
__main__
ntpath
nt
sys
__builtin__
site
signal
UserDict
stat</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
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<td width="12" valign="top" align="left"><a href="#fileinfo.modules.1.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">The <tt class="filename">sys</tt> module contains system-level information, such as the version of <span class="application">Python</span> you're running (<tt class="literal"><tt class="filename">sys</tt>.version</tt> or <tt class="literal"><tt class="filename">sys</tt>.version_info</tt>), and system-level options such as the maximum allowed recursion depth (<tt class="literal"><tt class="filename">sys</tt>.getrecursionlimit()</tt> and <tt class="literal"><tt class="filename">sys</tt>.setrecursionlimit()</tt>).
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<td width="12" valign="top" align="left"><a href="#fileinfo.modules.1.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="literal"><tt class="filename">sys</tt>.modules</tt> is a dictionary containing all the modules that have ever been imported since <span class="application">Python</span> was started; the key is the module name, the value is the module object. Note that this is more than just the modules <span class="emphasis"><em>your</em></span> program has imported. <span class="application">Python</span> preloads some modules on startup, and if you're using a <span class="application">Python</span> <span class="acronym">IDE</span>, <tt class="literal"><tt class="filename">sys</tt>.modules</tt> contains all the modules imported by all the programs you've run within the <span class="acronym">IDE</span>.
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<p>This example demonstrates how to use <tt class="literal"><tt class="filename">sys</tt>.modules</tt>.
</p>
<div class="example"><a name="d0e15784"></a><h3 class="title">Example 6.13. Using <tt class="literal"><tt class="filename">sys</tt>.modules</tt></h3><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>import</span> fileinfo</span> <a name="fileinfo.modules.1.3"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>print</span> <span class='pystring'>'\n'</span>.join(sys.modules.keys())</span>
<span class="computeroutput">win32api
os.path
os
fileinfo
exceptions
__main__
ntpath
nt
sys
__builtin__
site
signal
UserDict
stat</span>
<tt class="prompt">>>> </tt><span class="userinput">fileinfo</span>
<span class="computeroutput"><module 'fileinfo' from 'fileinfo.pyc'></span>
<tt class="prompt">>>> </tt><span class="userinput">sys.modules[<span class='pystring'>"fileinfo"</span>]</span> <a name="fileinfo.modules.1.4"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput"><module 'fileinfo' from 'fileinfo.pyc'></span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
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<td width="12" valign="top" align="left"><a href="#fileinfo.modules.1.3"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
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<td valign="top" align="left">As new modules are imported, they are added to <tt class="literal"><tt class="filename">sys</tt>.modules</tt>. This explains why importing the same module twice is very fast: <span class="application">Python</span> has already loaded and cached the module in <tt class="literal"><tt class="filename">sys</tt>.modules</tt>, so importing the second time is simply a dictionary lookup.
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<td valign="top" align="left">Given the name (as a string) of any previously-imported module, you can get a reference to the module itself through the <tt class="literal"><tt class="filename">sys</tt>.modules</tt> dictionary.
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<p>The next example shows how to use the <tt class="literal">__module__</tt> class attribute with the <tt class="literal"><tt class="filename">sys</tt>.modules</tt> dictionary to get a reference to the module in which a class is defined.
</p>
<div class="example"><a name="d0e15859"></a><h3 class="title">Example 6.14. The <tt class="literal">__module__</tt> Class Attribute
</h3><pre class="screen"><tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>from</span> fileinfo <span class='pykeyword'>import</span> MP3FileInfo</span>
<tt class="prompt">>>> </tt><span class="userinput">MP3FileInfo.__module__</span> <a name="fileinfo.modules.2.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput">'fileinfo'</span>
<tt class="prompt">>>> </tt><span class="userinput">sys.modules[MP3FileInfo.__module__]</span> <a name="fileinfo.modules.2.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput"><module 'fileinfo' from 'fileinfo.pyc'></span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
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<td width="12" valign="top" align="left"><a href="#fileinfo.modules.2.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
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<td valign="top" align="left">Every <span class="application">Python</span> class has a built-in <a href="../object_oriented_framework/class_attributes.html" title="5.8. Introducing Class Attributes">class attribute</a> <tt class="literal">__module__</tt>, which is the name of the module in which the class is defined.
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<td width="12" valign="top" align="left"><a href="#fileinfo.modules.2.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
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<td valign="top" align="left">Combining this with the <tt class="literal"><tt class="filename">sys</tt>.modules</tt> dictionary, you can get a reference to the module in which a class is defined.
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<p>Now you're ready to see how <tt class="literal"><tt class="filename">sys</tt>.modules</tt> is used in <tt class="filename">fileinfo.py</tt>, the sample program introduced in <a href="../object_oriented_framework/index.html">Chapter 5</a>. This example shows that portion of the code.
</p>
<div class="example"><a name="d0e15923"></a><h3 class="title">Example 6.15. <tt class="literal"><tt class="filename">sys</tt>.modules</tt> in <tt class="filename">fileinfo.py</tt></h3><pre class="programlisting">
<span class='pykeyword'>def</span><span class='pyclass'> getFileInfoClass</span>(filename, module=sys.modules[FileInfo.__module__]): <a name="fileinfo.modules.3.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class='pystring'>"get file info class from filename extension"</span>
subclass = <span class='pystring'>"%sFileInfo"</span> % os.path.splitext(filename)[1].upper()[1:] <a name="fileinfo.modules.3.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class='pykeyword'>return</span> hasattr(module, subclass) <span class='pykeyword'>and</span> getattr(module, subclass) <span class='pykeyword'>or</span> FileInfo <a name="fileinfo.modules.3.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></pre><div class="calloutlist">
<table border="0" summary="Callout list">
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<td width="12" valign="top" align="left"><a href="#fileinfo.modules.3.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">This is a function with two arguments; <tt class="varname">filename</tt> is required, but <tt class="varname">module</tt> is <a href="../power_of_introspection/optional_arguments.html" title="4.2. Using Optional and Named Arguments">optional</a> and defaults to the module that contains the <tt class="classname">FileInfo</tt> class. This looks inefficient, because you might expect <span class="application">Python</span> to evaluate the <tt class="literal"><tt class="filename">sys</tt>.modules</tt> expression every time the function is called. In fact, <span class="application">Python</span> evaluates default expressions only once, the first time the module is imported. As you'll see later, you never call this
function with a <tt class="varname">module</tt> argument, so <tt class="varname">module</tt> serves as a function-level constant.
</td>
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<td width="12" valign="top" align="left"><a href="#fileinfo.modules.3.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">You'll plow through this line later, after you dive into the <tt class="filename">os</tt> module. For now, take it on faith that <tt class="varname">subclass</tt> ends up as the name of a class, like <tt class="classname">MP3FileInfo</tt>.
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<td width="12" valign="top" align="left"><a href="#fileinfo.modules.3.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">You already know about <a href="../power_of_introspection/getattr.html" title="4.4. Getting Object References With getattr"><tt class="function">getattr</tt></a>, which gets a reference to an object by name. <tt class="function">hasattr</tt> is a complementary function that checks whether an object has a particular attribute; in this case, whether a module has
a particular class (although it works for any object and any attribute, just like <tt class="function">getattr</tt>). In English, this line of code says, “<span class="quote">If this module has the class named by <tt class="varname">subclass</tt> then return it, otherwise return the base class <tt class="classname">FileInfo</tt>.</span>”
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<div class="furtherreading">
<h3>Further Reading on Modules</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>.
</li>
<li><a href="http://www.python.org/doc/current/lib/"><i class="citetitle"><span class="application">Python</span> Library Reference</i></a> documents the <a href="http://www.python.org/doc/current/lib/module-sys.html"><tt class="filename">sys</tt></a> module.
</li>
</ul>
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