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All Things Pythonic
Bob Martin's Agile Programming book
by Guido van van Rossum
February 3, 2004
Half book review, half Python-vs-static-languages musings


I wrote this half a year ago, planning to go back to the book for some fact checking before posting it. Then I accepted a new job, the book got packed, and I was too busy to blog.

I'm accepting the status quo: I'm still too busy, the book is in a box that I still haven't unpacked, so I'm just posting this unfinished. I'm sure commenters will correct my facts if they need correcting. ;-)


I enjoyed reading Bob Martin's "Agile Programming". I haven't quite finished it, but I'm confident I can start writing this review now, while I have a few moments to myself. There's no Internet above the Atlantic Ocean yet to distract me with email, and my family is sadly remaining at home. And I'm procrastinating writing my keynote for EuroPython :). (The second pass of this text was done on a plane from Portland to San Francisco, returning from OSCON 2003 -- see my previous blog entry.)

I really liked the book, although it was very different from what I had expected. Well, what had I expected? Maybe more formal writing; Bob's writing (this may come as no surprise to his fans) is incredibly informal, despite the many footnotes (half of which contain jokes) and references (mostly to the "Gang Of Four" patterns book and Bob's own previous work). Maybe I had expected the code samples to be more sophisticated. Maybe I had expected fewer typos; I expect Bob's enough of a control freak to produce camera-ready copy, which must limit the effectiveness of a publisher's editors and proofreaders against slippery fingers.

But the typos didn't distract from the book's message. It starts with a clear manifesto, proclaiming (amongst others) that programmers aren't to be treated like exchangeable production workers. While touting various "agile" development methods, Bob is clearly partial to Extreme Programming, probably the most outgoing of the agile methods. (I would call it my favorite except I'm pretty ignorant of the others, so that endorsement wouldn't mean much.)

There is a lot of code in the book. I haven't read all of it, but I've skimmed it all and read much of it. I'd say two-third is Java code, the rest is C++. Python gets a mention or two, together with Smalltalk and Ruby (apparently Bob's current favorite) as examples of "more dynamic languages" where some things are different or easier.

Seeing all that Java and C++ reminded me of how verbose these languages often are! After reading Bob's very entertaining chapter of how he and another Bob wrote a scoring program for bowling, I wrote a similar program in Python in 75 minutes and 85 lines, plus 150 lines of unit testing code. [XXX insert comparison in #lines.] My program used a much simpler data structure (a list of lists, each sublist representing a frame) and didn't follow the flow of Bob's program; I simply did what I thought was the best way to solve the problem in Python. I don't think that reading Bob's program did much to influence my version, except that I used it to learn the scoring rules for bowling (not being a native American, I had only the vaguest idea of the difference between a strike and a spare before reading this chapter).

But in a sense, the verbosity of Java and C++ help the book; the method of test-driven design practiced throughout the book doesn't work for very small programs, and the bowling scoring program in Python would perhaps have been too small to make a good example (although the quirkiness of the rules of bowling definitely help).

The large volume of code in the book is important for Bob's story: there's a lot of it, often shown in various stages of development, from the first skeleton version, that barely passes the most trivial unit test, to the final, complete version, with full unit tests. This is an exellent illustration of test-driven design, although I still find it hard to believe that it really works like that in real life, when practiced in the extreme form shown in the book.

I am a firm believer in writing unit tests; in a recent mid-size programming project, adding a fairly elaborate new feature to Zope 3, I found time after time that the remaining bugs in the code coincided with places where I had cut corners in the unit tests. But the unit tests played only a small role in the design of my code. There was a solid high-level design on paper, done months before I got the project, and this design for the most part worked well. It wasn't grand enough to call it "big design up front" (which has a bad rep in the XP world); most programming details weren't fixed by the design and in fact I wrote large parts of the code twice, first to get a feel for the "lay of the land", and then another time to "get it right". Only the second version of the code had solid unit tests, which were essential to get all the code details right -- but they didn't influence the design much, which was formed by thinking over the problem after writing the first prototype. To be honest, there was an even earlier prototype by someone else, which I ended up mostly throwing away, but which probably informed the design with which I started.

Besides writing about XP and test-driven design, Bob writes about a number of programming patterns. Most of these are repeats of Gang-of-Four patterns, some come from Bob's own patterns book. I find Bob's approach refreshing: while I own the GoF book, I haven't read it cover-to-cover, and only occasionally used it as a reference. Bob's treatment is lighter, but more importantly, he repeatedly warns for mindless use of patterns. He explains how some patterns can easily lead to overengineered code, and shows examples of lighter alternatives (usually different patterns from the same sources).

But more than test-driven design or patterns, for me the centerpiece of the book are the various principles of agile development. There is the Liskov Substitution Principle, a reminder about the intent of subclassing; the Single Responsibility Principle, which can be used to decide when it's better to split a class into two; the Open-Closed Principle, stating that a class should be open to extension but closed to change (since every change to a class requires relinking all code that depends on it; not quite true in Python, but still a useful principle).

Finally there's the Dependency Inversion Principle, which reminds us that rather than having the higher-level classes depend on the lower-level classes, like we used to do it, we should have the higher-level classes define interfaces upon which the lower-level classes depend. This principle, like several of the others, is designed to decouple different parts of a program, so there's less need for relinking. Again, this is less important for Python. In C++ or Java this change usually means that you have to create an interface describing the concrete low-level class, rewrite the higher-level class to take an instance implementing that interface, and rewrite the lower-level class to implement the interface.

Once this is done, the low-level class can change without affecting the higher-level class (which only depends on the interface), as opposed to before the change, when every change in the lower-level class required relinking the higher-level class. Why is this bad? The higher-level class doesn't change that much, but the lower-level class is where most of the action is, and hence much of the churn during development. Surely reducing the dependencies on a high-churn class must be a worthy goal!

But in Python, the implementation of this cunning plan is almost too simple to deserve a Principle with a capital P. Since in Python, interfaces (we tend to call them protocols) are usually only in the programmer's head, all we have to do is to make sure that the higher-level class doesn't directly instantiate the lower-level class, but gets an instance (or instances, as the case may be; or perhaps a factory function) passed in. Then we can change the lower-level class as long as the signatures of the methods used by the higher-level class don't change (this is Python's equivalent of keeping the interface fixed); and we can even substituting an entirely different class as long as it has the same set of methods.


PS. (back to today, February 2004): does anybody else think that Ant is a poor excuse for a build tool? Some of my top complaints:

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About the Blogger

Guido van Rossum is the creator of Python, one of the major programming languages on and off the web. The Python community refers to him as the BDFL (Benevolent Dictator For Life), a title straight from a Monty Python skit. He moved from the Netherlands to the USA in 1995, where he met his wife. Until July 2003 they lived in the northern Virginia suburbs of Washington, DC with their son Orlijn, who was born in 2001. They then moved to Silicon Valley where Guido now works for Google (spending 50% of his time on Python!).

This weblog entry is Copyright © 2004 Guido van van Rossum. All rights reserved.

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