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<h2 class="title"><a name="regression.map"></a>16.4. Mapping lists revisited
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<div class="abstract">
<p>You're already familiar with using <a href="../native_data_types/mapping_lists.html" title="3.6. Mapping Lists">list comprehensions</a> to map one list into another. There is another way to accomplish the same thing, using the built-in <tt class="function">map</tt> function. It works much the same way as the <a href="filtering_lists.html" title="16.3. Filtering lists revisited"><tt class="function">filter</tt></a> function.
</p>
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<div class="example"><a name="d0e36003"></a><h3 class="title">Example 16.10. Introducing <tt class="function">map</tt></h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>def</span><span class='pyclass'> double</span>(n):</span>
<tt class="prompt">... </tt>return n*2
<tt class="prompt">... </tt>
<tt class="prompt">>>> </tt><span class="userinput">li = [1, 2, 3, 5, 9, 10, 256, -3]</span>
<tt class="prompt">>>> </tt><span class="userinput">map(double, li)</span> <a name="regression.map.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput">[2, 4, 6, 10, 18, 20, 512, -6]</span>
<tt class="prompt">>>> </tt><span class="userinput">[double(n) <span class='pykeyword'>for</span> n <span class='pykeyword'>in</span> li]</span> <a name="regression.map.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
<span class="computeroutput">[2, 4, 6, 10, 18, 20, 512, -6]</span>
<tt class="prompt">>>> </tt><span class="userinput">newlist = []</span>
<tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>for</span> n <span class='pykeyword'>in</span> li:</span> <a name="regression.map.1.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
<tt class="prompt">... </tt><span class="userinput">newlist.append(double(n))</span>
<tt class="prompt">... </tt>
<tt class="prompt">>>> </tt><span class="userinput">newlist</span>
<span class="computeroutput">[2, 4, 6, 10, 18, 20, 512, -6]</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
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<td width="12" valign="top" align="left"><a href="#regression.map.1.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left"><tt class="function">map</tt> takes a function and a list<sup>[<a name="d0e36079" href="#ftn.d0e36079">8</a>]</sup> and returns a new list by calling the function with each element of the list in order. In this case, the function simply
multiplies each element by 2.
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<td width="12" valign="top" align="left"><a href="#regression.map.1.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">You could accomplish the same thing with a list comprehension. List comprehensions were first introduced in <span class="application">Python</span> 2.0; <tt class="function">map</tt> has been around forever.
</td>
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<td width="12" valign="top" align="left"><a href="#regression.map.1.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">You could, if you insist on thinking like a <span class="application">Visual Basic</span> programmer, use a <tt class="literal">for</tt> loop to accomplish the same thing.
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<div class="example"><a name="d0e36107"></a><h3 class="title">Example 16.11. <tt class="function">map</tt> with lists of mixed datatypes
</h3><pre class="screen">
<tt class="prompt">>>> </tt><span class="userinput">li = [5, <span class='pystring'>'a'</span>, (2, <span class='pystring'>'b'</span>)]</span>
<tt class="prompt">>>> </tt><span class="userinput">map(double, li)</span> <a name="regression.map.2.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
<span class="computeroutput">[10, 'aa', (2, 'b', 2, 'b')]</span></pre><div class="calloutlist">
<table border="0" summary="Callout list">
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<td width="12" valign="top" align="left"><a href="#regression.map.2.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">As a side note, I'd like to point out that <tt class="function">map</tt> works just as well with lists of mixed datatypes, as long as the function you're using correctly handles each type. In this
case, the <tt class="function">double</tt> function simply multiplies the given argument by 2, and <span class="application">Python</span> Does The Right Thing depending on the datatype of the argument. For integers, this means actually multiplying it by 2; for
strings, it means concatenating the string with itself; for tuples, it means making a new tuple that has all of the elements
of the original, then all of the elements of the original again.
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<p>All right, enough play time. Let's look at some real code.</p>
<div class="example"><a name="d0e36143"></a><h3 class="title">Example 16.12. <tt class="function">map</tt> in <tt class="filename">regression.py</tt></h3><pre class="programlisting">
filenameToModuleName = <span class='pykeyword'>lambda</span> f: os.path.splitext(f)[0] <a name="regression.map.3.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
moduleNames = map(filenameToModuleName, files) <a name="regression.map.3.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></pre><div class="calloutlist">
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<td width="12" valign="top" align="left"><a href="#regression.map.3.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">As you saw in <a href="../power_of_introspection/lambda_functions.html" title="4.7. Using lambda Functions">Section 4.7, “Using lambda Functions”</a>, <tt class="literal">lambda</tt> defines an inline function. And as you saw in <a href="../file_handling/os_module.html#splittingpathnames.example" title="Example 6.17. Splitting Pathnames">Example 6.17, “Splitting Pathnames”</a>, <tt class="function">os.path.splitext</tt> takes a filename and returns a tuple <tt class="literal">(<i class="replaceable">name</i>, <i class="replaceable">extension</i>)</tt>. So <tt class="function">filenameToModuleName</tt> is a function which will take a filename and strip off the file extension, and return just the name.
</td>
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<td width="12" valign="top" align="left"><a href="#regression.map.3.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
</td>
<td valign="top" align="left">Calling <tt class="function">map</tt> takes each filename listed in <tt class="varname">files</tt>, passes it to the function <tt class="function">filenameToModuleName</tt>, and returns a list of the return values of each of those function calls. In other words, you strip the file extension off
of each filename, and store the list of all those stripped filenames in <tt class="varname">moduleNames</tt>.
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<p>As you'll see in the rest of the chapter, you can extend this type of data-centric thinking all the way to the final goal,
which is to define and execute a single test suite that contains the tests from all of those individual test suites.
</p>
<div class="footnotes">
<h3 class="footnotetitle">Footnotes</h3>
<div class="footnote">
<p><sup>[<a name="ftn.d0e36079" href="#d0e36079">8</a>] </sup>Again, I should point out that <tt class="function">map</tt> can take a list, a tuple, or any object that acts like a sequence. See previous footnote about <tt class="function">filter</tt>.
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