16.5. Data-centric programming

By now you're probably scratching your head wondering why this is better than using for loops and straight function calls. And that's a perfectly valid question. Mostly, it's a matter of perspective. Using map and filter forces you to center your thinking around your data.

In this case, you started with no data at all; the first thing you did was get the directory path 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.

However, you knew you didn't care about all of those files, only the ones that were actually test suites. You had too much data, so you needed to filter it. How did you know which data to keep? You needed a test to decide, so you defined one and passed it to the filter 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.

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 in the wrong format. 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.

Instead of filter, you could have used a for loop with an if statement. Instead of map, you could have used a for loop with a function call. But using for 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 “is this file a test suite?” 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 for loop and an if statement and manually calling append 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 Python do the rest of that work for us?

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 Python 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.

I resisted list comprehensions when I first learned Python, and I resisted filter and map even longer. I insisted on making my life more difficult, sticking to the familiar way of for loops and if statements and step-by-step code-centric programming. And my Python programs looked a lot like Visual Basic 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.

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.