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<h2 class="title"><a name="regression.datacentric"></a>16.5. Data-centric programming
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<div class="abstract">
<p>By now you're probably scratching your head wondering why this is better than using <tt class="literal">for</tt> loops and straight function calls. And that's a perfectly valid question. Mostly, it's a matter of perspective. Using
<tt class="function">map</tt> and <tt class="function">filter</tt> forces you to center your thinking around your data.
</p>
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<p>In this case, you started with no data at all; the first thing you did was <a href="finding_the_path.html" title="16.2. Finding the path">get the directory path</a> of the current script, and got a list of files in that directory. That was the bootstrap, and it gave you real data to work
with: a list of filenames.
</p>
<p>However, you knew you didn't care about all of those files, only the ones that were actually test suites. You had <span class="emphasis"><em>too much data</em></span>, so you needed to <tt class="function">filter</tt> it. How did you know which data to keep? You needed a test to decide, so you defined one and passed it to the <tt class="function">filter</tt> function. In this case you used a regular expression to decide, but the concept would be the same regardless of how you
constructed the test.
</p>
<p>Now you had the filenames of each of the test suites (and only the test suites, since everything else had been filtered out),
but you really wanted module names instead. You had the right amount of data, but it was <span class="emphasis"><em>in the wrong format</em></span>. So you defined a function that would transform a single filename into a module name, and you mapped that function onto
the entire list. From one filename, you can get a module name; from a list of filenames, you can get a list of module names.
</p>
<p>Instead of <tt class="function">filter</tt>, you could have used a <tt class="literal">for</tt> loop with an <tt class="literal">if</tt> statement. Instead of <tt class="function">map</tt>, you could have used a <tt class="literal">for</tt> loop with a function call. But using <tt class="literal">for</tt> loops like that is busywork. At best, it simply wastes time; at worst, it introduces obscure bugs. For instance, you need
to figure out how to test for the condition “<span class="quote">is this file a test suite?</span>” anyway; that's the application-specific logic, and no language can write that for us. But once you've figured that out,
do you really want go to all the trouble of defining a new empty list and writing a <tt class="literal">for</tt> loop and an <tt class="literal">if</tt> statement and manually calling <tt class="function">append</tt> to add each element to the new list if it passes the condition and then keeping track of which variable holds the new filtered
data and which one holds the old unfiltered data? Why not just define the test condition, then let <span class="application">Python</span> do the rest of that work for us?
</p>
<p>Oh sure, you could try to be fancy and delete elements in place without creating a new list. But you've been burned by that
before. Trying to modify a data structure that you're looping through can be tricky. You delete an element, then loop to
the next element, and suddenly you've skipped one. Is <span class="application">Python</span> one of the languages that works that way? How long would it take you to figure it out? Would you remember for certain whether
it was safe the next time you tried? Programmers spend so much time and make so many mistakes dealing with purely technical
issues like this, and it's all pointless. It doesn't advance your program at all; it's just busywork.
</p>
<p>I resisted list comprehensions when I first learned <span class="application">Python</span>, and I resisted <tt class="function">filter</tt> and <tt class="function">map</tt> even longer. I insisted on making my life more difficult, sticking to the familiar way of <tt class="literal">for</tt> loops and <tt class="literal">if</tt> statements and step-by-step code-centric programming. And my <span class="application">Python</span> programs looked a lot like <span class="application">Visual Basic</span> programs, detailing every step of every operation in every function. And they had all the same types of little problems
and obscure bugs. And it was all pointless.
</p>
<p>Let it all go. Busywork code is not important. Data is important. And data is not difficult. It's only data. If you have
too much, filter it. If it's not what you want, map it. Focus on the data; leave the busywork behind.
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