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| 57 | <div>
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| 58 | <div>
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| 59 | <h2 class="title"><a name="regression.filter"></a>16.3. Filtering lists revisited
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| 60 | </h2>
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| 61 | </div>
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| 62 | </div>
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| 63 | <div></div>
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| 64 | </div>
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| 65 | <div class="abstract">
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| 66 | <p>You're already familiar with <a href="../power_of_introspection/filtering_lists.html" title="4.5. Filtering Lists">using list comprehensions to filter lists</a>. There is another way to accomplish this same thing, which some people feel is more expressive.
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| 67 | </p>
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| 68 | </div>
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| 69 | <p><span class="application">Python</span> has a built-in <tt class="function">filter</tt> function which takes two arguments, a function and a list, and returns a list.<sup>[<a name="d0e35697" href="#ftn.d0e35697">7</a>]</sup> The function passed as the first argument to <tt class="function">filter</tt> must itself take one argument, and the list that <tt class="function">filter</tt> returns will contain all the elements from the list passed to <tt class="function">filter</tt> for which the function passed to <tt class="function">filter</tt> returns true.
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| 70 | </p>
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| 71 | <p>Got all that? It's not as difficult as it sounds.</p>
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| 72 | <div class="example"><a name="d0e35724"></a><h3 class="title">Example 16.7. Introducing <tt class="function">filter</tt></h3><pre class="screen">
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| 73 | <tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>def</span><span class='pyclass'> odd</span>(n):</span> <a name="regression.filter.1.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
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| 74 | <tt class="prompt">... </tt><span class="userinput"><span class='pykeyword'>return</span> n % 2</span>
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| 75 | <tt class="prompt">... </tt>
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| 76 | <tt class="prompt">>>> </tt><span class="userinput">li = [1, 2, 3, 5, 9, 10, 256, -3]</span>
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| 77 | <tt class="prompt">>>> </tt><span class="userinput">filter(odd, li)</span> <a name="regression.filter.1.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
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| 78 | <span class="computeroutput">[1, 3, 5, 9, -3]</span>
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| 79 | <tt class="prompt">>>> </tt><span class="userinput">[e <span class='pykeyword'>for</span> e <span class='pykeyword'>in</span> li <span class='pykeyword'>if</span> odd(e)]</span> <a name="regression.filter.1.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12">
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| 80 | <tt class="prompt">>>> </tt><span class="userinput">filteredList = []</span>
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| 81 | <tt class="prompt">>>> </tt><span class="userinput"><span class='pykeyword'>for</span> n <span class='pykeyword'>in</span> li:</span> <a name="regression.filter.1.4"></a><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12">
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| 82 | <tt class="prompt">... </tt><span class="userinput"><span class='pykeyword'>if</span> odd(n):</span>
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| 83 | <tt class="prompt">... </tt><span class="userinput"> filteredList.append(n)</span>
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| 84 | <tt class="prompt">... </tt>
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| 85 | <tt class="prompt">>>> </tt><span class="userinput">filteredList</span>
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| 86 | <span class="computeroutput">[1, 3, 5, 9, -3]</span></pre><div class="calloutlist">
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| 87 | <table border="0" summary="Callout list">
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| 88 | <tr>
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| 89 | <td width="12" valign="top" align="left"><a href="#regression.filter.1.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
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| 90 | </td>
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| 91 | <td valign="top" align="left"><tt class="function">odd</tt> uses the built-in mod function “<span class="quote"><tt class="literal">%</tt></span>” to return <tt class="constant">True</tt> if <tt class="varname">n</tt> is odd and <tt class="constant">False</tt> if <tt class="varname">n</tt> is even.
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| 92 | </td>
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| 93 | </tr>
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| 94 | <tr>
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| 95 | <td width="12" valign="top" align="left"><a href="#regression.filter.1.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
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| 96 | </td>
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| 97 | <td valign="top" align="left"><tt class="function">filter</tt> takes two arguments, a function (<tt class="function">odd</tt>) and a list (<tt class="varname">li</tt>). It loops through the list and calls <tt class="function">odd</tt> with each element. If <tt class="function">odd</tt> returns a true value (remember, any non-zero value is true in <span class="application">Python</span>), then the element is included in the returned list, otherwise it is filtered out. The result is a list of only the odd
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| 98 | numbers from the original list, in the same order as they appeared in the original.
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| 99 | </td>
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| 100 | </tr>
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| 101 | <tr>
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| 102 | <td width="12" valign="top" align="left"><a href="#regression.filter.1.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
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| 103 | </td>
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| 104 | <td valign="top" align="left">You could accomplish the same thing using list comprehensions, as you saw in <a href="../power_of_introspection/filtering_lists.html" title="4.5. Filtering Lists">Section 4.5, “Filtering Lists”</a>.
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| 105 | </td>
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| 106 | </tr>
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| 107 | <tr>
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| 108 | <td width="12" valign="top" align="left"><a href="#regression.filter.1.4"><img src="../images/callouts/4.png" alt="4" border="0" width="12" height="12"></a>
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| 109 | </td>
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| 110 | <td valign="top" align="left">You could also accomplish the same thing with a <tt class="literal">for</tt> loop. Depending on your programming background, this may seem more “<span class="quote">straightforward</span>”, but functions like <tt class="function">filter</tt> are much more expressive. Not only is it easier to write, it's easier to read, too. Reading the <tt class="literal">for</tt> loop is like standing too close to a painting; you see all the details, but it may take a few seconds to be able to step
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| 111 | back and see the bigger picture: “<span class="quote">Oh, you're just filtering the list!</span>”
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| 112 | </td>
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| 113 | </tr>
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| 114 | </table>
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| 115 | </div>
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| 116 | </div>
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| 117 | <div class="example"><a name="d0e35864"></a><h3 class="title">Example 16.8. <tt class="function">filter</tt> in <tt class="filename">regression.py</tt></h3><pre class="programlisting">
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| 118 | files = os.listdir(path) <a name="regression.filter.2.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12">
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| 119 | test = re.compile(<span class='pystring'>"test\.py$"</span>, re.IGNORECASE) <a name="regression.filter.2.2"></a><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12">
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| 120 | files = filter(test.search, files) <a name="regression.filter.2.3"></a><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></pre><div class="calloutlist">
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| 121 | <table border="0" summary="Callout list">
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| 122 | <tr>
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| 123 | <td width="12" valign="top" align="left"><a href="#regression.filter.2.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
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| 124 | </td>
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| 125 | <td valign="top" align="left">As you saw in <a href="finding_the_path.html" title="16.2. Finding the path">Section 16.2, “Finding the path”</a>, <tt class="varname">path</tt> may contain the full or partial pathname of the directory of the currently running script, or it may contain an empty string
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| 126 | if the script is being run from the current directory. Either way, <tt class="varname">files</tt> will end up with the names of the files in the same directory as this script you're running.
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| 127 | </td>
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| 128 | </tr>
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| 129 | <tr>
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| 130 | <td width="12" valign="top" align="left"><a href="#regression.filter.2.2"><img src="../images/callouts/2.png" alt="2" border="0" width="12" height="12"></a>
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| 131 | </td>
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| 132 | <td valign="top" align="left">This is a compiled regular expression. As you saw in <a href="../refactoring/refactoring.html" title="15.3. Refactoring">Section 15.3, “Refactoring”</a>, if you're going to use the same regular expression over and over, you should compile it for faster performance. The compiled
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| 133 | object has a <tt class="function">search</tt> method which takes a single argument, the string to search. If the regular expression matches the string, the <tt class="function">search</tt> method returns a <tt class="classname">Match</tt> object containing information about the regular expression match; otherwise it returns <tt class="literal">None</tt>, the <span class="application">Python</span> null value.
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| 134 | </td>
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| 135 | </tr>
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| 136 | <tr>
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| 137 | <td width="12" valign="top" align="left"><a href="#regression.filter.2.3"><img src="../images/callouts/3.png" alt="3" border="0" width="12" height="12"></a>
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| 138 | </td>
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| 139 | <td valign="top" align="left">For each element in the <tt class="varname">files</tt> list, you're going to call the <tt class="function">search</tt> method of the compiled regular expression object, <tt class="varname">test</tt>. If the regular expression matches, the method will return a <tt class="classname">Match</tt> object, which <span class="application">Python</span> considers to be true, so the element will be included in the list returned by <tt class="function">filter</tt>. If the regular expression does not match, the <tt class="function">search</tt> method will return <tt class="literal">None</tt>, which <span class="application">Python</span> considers to be false, so the element will not be included.
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| 140 | </td>
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| 141 | </tr>
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| 142 | </table>
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| 143 | </div>
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| 144 | </div>
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| 145 | <p><b>Historical note. </b>Versions of <span class="application">Python</span> prior to 2.0 did not have <a href="../native_data_types/mapping_lists.html" title="3.6. Mapping Lists">list comprehensions</a>, so you couldn't <a href="../power_of_introspection/filtering_lists.html" title="4.5. Filtering Lists">filter using list comprehensions</a>; the <tt class="function">filter</tt> function was the only game in town. Even with the introduction of list comprehensions in 2.0, some people still prefer the
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| 146 | old-style <tt class="function">filter</tt> (and its companion function, <tt class="function">map</tt>, which you'll see later in this chapter). Both techniques work at the moment, so which one you use is a matter of style.
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| 147 | There is discussion that <tt class="function">map</tt> and <tt class="function">filter</tt> might be deprecated in a future version of <span class="application">Python</span>, but no decision has been made.
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| 148 | </p>
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| 149 | <div class="example"><a name="d0e35972"></a><h3 class="title">Example 16.9. Filtering using list comprehensions instead</h3><pre class="programlisting">
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| 150 | files = os.listdir(path)
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| 151 | test = re.compile(<span class='pystring'>"test\.py$"</span>, re.IGNORECASE)
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| 152 | files = [f <span class='pykeyword'>for</span> f <span class='pykeyword'>in</span> files <span class='pykeyword'>if</span> test.search(f)] <a name="regression.filter.3.1"></a><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></pre><div class="calloutlist">
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| 153 | <table border="0" summary="Callout list">
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| 154 | <tr>
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| 155 | <td width="12" valign="top" align="left"><a href="#regression.filter.3.1"><img src="../images/callouts/1.png" alt="1" border="0" width="12" height="12"></a>
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| 156 | </td>
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| 157 | <td valign="top" align="left">This will accomplish exactly the same result as using the <tt class="function">filter</tt> function. Which way is more expressive? That's up to you.
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| 158 | </td>
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| 159 | </tr>
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| 160 | </table>
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| 161 | </div>
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| 162 | </div>
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| 163 | <div class="footnotes">
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| 164 | <h3 class="footnotetitle">Footnotes</h3>
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| 165 | <div class="footnote">
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| 166 | <p><sup>[<a name="ftn.d0e35697" href="#d0e35697">7</a>] </sup>Technically, the second argument to <tt class="function">filter</tt> can be any sequence, including lists, tuples, and custom classes that act like lists by defining the <tt class="function">__getitem__</tt> special method. If possible, <tt class="function">filter</tt> will return the same datatype as you give it, so filtering a list returns a list, but filtering a tuple returns a tuple.
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| 167 | </p>
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| 168 | </div>
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| 169 | </div>
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| 170 | </div>
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| 174 | <td width="30%" align="center"><br> <span class="divider">|</span> <a href="index.html#regression.divein" title="16.1. Diving in">1</a> <span class="divider">|</span> <a href="finding_the_path.html" title="16.2. Finding the path">2</a> <span class="divider">|</span> <span class="thispage">3</span> <span class="divider">|</span> <a href="mapping_lists.html" title="16.4. Mapping lists revisited">4</a> <span class="divider">|</span> <a href="data_centric.html" title="16.5. Data-centric programming">5</a> <span class="divider">|</span> <a href="dynamic_import.html" title="16.6. Dynamically importing modules">6</a> <span class="divider">|</span> <a href="all_together.html" title="16.7. Putting it all together">7</a> <span class="divider">|</span> <a href="summary.html" title="16.8. Summary">8</a> <span class="divider">|</span>
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