Python in a Nutshell, 2nd Edition

Optimization

“First make it work. Then make it right. Then make it fast.” This quotation, often with slight variations, is widely known as “the golden rule of programming.” As far as I’ve been able to ascertain, the quotation is by Kent Beck, who credits his father with it. Being widely known makes the principle no less important, particularly because it’s more honored in the breach than in the observance. A negative form, slightly exaggerated for emphasis, is in a quotation by Don Knuth (who credits Hoare with it): “Premature optimization is the root of all evil in programming.”

Optimization is premature if your code is not working yet, or if you’re not sure about what, exactly, your code should be doing (since then you cannot be sure if it’s working). First make it work. Optimization is also premature if your code is working but you are not satisfied with the overall architecture and design. Remedy structural flaws before worrying about optimization: first make it work, then make it right. These first two steps are not optional; working, well-architected code is always a must.

In contrast, you don’t always need to make it fast. Benchmarks may show that your code’s performance is already acceptable after the first two steps. When performance is not acceptable, profiling often shows that all performance issues are in a small part of the code, perhaps 10 to 20 percent of the code where your program spends 80 or 90 percent of the time. Such performance-crucial regions of your code are ...