Debugging and Profiling

"If debugging is the process of removing software bugs, then programming must be the process of putting them in."

– Edsger W. Dijkstra

In the life of a professional coder, debugging and troubleshooting take up a significant amount of time. Even if you work on the most beautiful code base ever written by a human, there will still be bugs in it; that is guaranteed. We spend an awful lot of time reading other people's code and, in our opinion, a good software developer is someone who keeps an eye out for potential bugs, even when they're reading code that is not reported to be wrong or buggy.

Being able to debug code efficiently and quickly is a skill that every coder needs to keep improving. Like testing, debugging is a skill that is best learned through experience. There are guidelines you can follow, but there is no magic book that will teach you everything you need to know in order to become good at this.

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Debugging techniques

In this part, we'll introduce you to some of the techniques we use most often. This is not an exhaustive list, but it should give you some useful ideas for where to start when debugging your own Python code.

Debugging with print

The key to understanding any bug is to understand what your code is doing at the point where the bug occurs. For this reason, we'll be looking at a few different techniques for inspecting the state of a program while it is running.

Probably the easiest technique of all is to add print() calls at various points in your code. This allows you to easily see which parts of your code are executed, and what the values of key variables are at different points during execution. For example, if you are developing a Django website and what happens on a page is not what you would expect, you can fill the view with prints and keep an eye on the console while you reload the page.

There are several drawbacks and limitations...

Troubleshooting guidelines

In this short section, we'd like to give you a few tips that come from our troubleshooting experience.

Where to inspect

Our first suggestion concerns where to place your debugging breakpoints. Regardless of whether you are using print(), a custom function, pdb, or logging, you still have to choose where to place the calls that provide you with the information. Some places are definitely better than others, and there are ways to handle the debugging progression that are better than others.

We normally avoid placing a breakpoint inside an if clause. If the branch containing the breakpoint is not executed, we lose the chance to get the information we wanted. Sometimes it can be difficult to reproduce a bug, or it may take a while for your code to reach the breakpoint, so think carefully before placing them.

Another important thing is where to start. Imagine that you have 100 lines of code that handle your data. Data comes in at line 1,...

Profiling Python

Profiling means having the application run while keeping track of several different parameters, such as the number of times a function is called and the amount of time spent inside it.

Profiling is closely related to debugging. Although the tools and processes used are quite different, both activities involve probing and analyzing your code to understand where the root of a problem lies and then making changes to fix it. The difference is that instead of incorrect output or crashing, the problem we are trying to solve is poor performance.

Sometimes profiling will point to where the performance bottleneck is, at which point you will need to use the debugging techniques we discussed earlier in this chapter to understand why a particular piece of code does not perform as well as it should. For example, faulty logic in a database query might result in loading thousands of rows from a table instead of just hundreds. Profiling might show you that a particular function...

Summary

In this short chapter, we looked at different techniques and suggestions for debugging, troubleshooting, and profiling our code. Debugging is an activity that is always part of a software developer's work, so it's important to be good at it.

If approached with the correct attitude, it can be fun and rewarding.

We explored techniques to inspect our code using custom functions, logging, debuggers, traceback information, profiling, and assertions. We saw simple examples of most of them and we also talked about some guidelines that will help when it comes to facing the fire.

Just remember always to stay calm and focused, and debugging will be much easier. This, too, is a skill that needs to be learned and it's the most important. An agitated and stressed mind cannot work properly, logically, and creatively, therefore, if you don't strengthen it, it will be hard for you to put all of your knowledge to good use. So, when facing a difficult bug, if...