3.2. 👩💻 Debugging¶
Before we dive into the nitty gritty details of debugging, here is a video to give you a flavor for what its like to be a programmer in the real world.
Programming is a complex process. Since it is done by human beings, errors may often occur. Programming errors are called bugs and the process of tracking them down and correcting them is called debugging. Some claim that in 1945, a dead moth caused a problem on relay number 70, panel F, of one of the first computers at Harvard, and the term bug has remained in use since. For more about this historic event, see first bug.
One of the most important skills you need to acquire to complete this book successfully is the ability to debug your programs. Debugging might be the most under-appreciated, and under-taught, skill in introductory computer science. For that reason we are introducing a series of “debugging interludes.” Debugging is a skill that you need to master over time, and some of the tips and tricks are specific to different aspects of Python programming. So look for additional Way of the Programmer interludes throughout the rest of this book.
Programming is an odd thing in a way. Here is why. As programmers we spend 99% of our time trying to get our program to work. We struggle, we stress, we spend hours deep in frustration trying to get our program to execute correctly. Then when we do get it going we celebrate, hand it in, and move on to the next homework assignment or programming task. But here is the secret, when you are successful, you are happy, your brain releases a bit of chemical that makes you feel good. You need to organize your programming so that you have lots of little successess. It turns out your brain doesn’t care all that much if you have successfully written hello world, or a fast fourier transform (trust me its hard) you still get that little release that makes you happy. When you are happy you want to go on and solve the next little problem. Essentially I’m telling you once again, start small, get something small working, and then add to it.
3.2.1. How to Avoid Debugging¶
Perhaps the most important lesson in debugging is that it is largely avoidable – if you work carefully.
Understand the Problem You must have a firm grasp on what you are trying to accomplish but not necessarily how to do it. You do not need to understand the entire problem. But you must understand at least a portion of it and what the program should do in a specific circumstance – what output should be produced for some given input. This will allow you to test your progress. You can then identify if a solution is correct or whether there remains work to do or bugs to fix. This is probably the single biggest piece of advice for programmers at every level.
Start Small It is tempting to sit down and crank out an entire program at once. But, when the program – inevitably – does not work, you have a myriad of options for things that might be wrong. Where to start? Where to look first? How to figure out what went wrong? I’ll get to that in the next section. So, start with something really small. Maybe just two lines and then make sure that runs. Hitting the run button is quick and easy. It gives you immediate feedback about whether what you have just done works or not. Another immediate benefit of having something small working is that you have something to turn in. Turning in a small, incomplete program, is almost always better than nothing.
Keep Improving It Once you have a small part of your program working, the next step is to figure out something small to add to it – how can you move closer to a correct solution. As you add to your program, you gain greater insight into the underlying problem you are trying to solve.
If you keep adding small pieces of the program one at a time, it is much easier to figure out what went wrong. (This of course means you must be able to recognize if there is an error. And that is done through testing.)
As long as you always test each new bit of code, it is most likely that any error is in the new code you have just added. Less new code means its easier to figure out where the problem is.
This notion of Get something working and keep improving it is a mantra that you can repeat throughout your career as a programmer. It’s a great way to avoid the frustrations mentioned above. Think of it this way. Every time you have a little success, your brain releases a tiny bit of chemical that makes you happy. So, you can keep yourself happy and make programming more enjoyable by creating lots of small victories for yourself.
The technique of start small and keep improving is the basis of Agile software development. This practice is used widely in the industry.
Ok, lets look at an example. Lets solve the problem posed in question 3 at the end of the Simple Python Data chapter. Ask the user for the time now (in hours 0 – 23), and ask for the number of hours to wait. Your program should output what the time will be on the clock when the alarm goes off.
So, where to start? The problem requires two pieces of input from the user, so lets start there and make sure we can get the data we need.
If you haven’t yet, click Run: get in the habit of checking whether small things are working before you go on.
So far so good. Now lets take the next step. We need to figure out what the time will be after
wait_time number of hours. A good first approximation to that is to simply add
current_time and print out the result. So lets try that.
Hmm, when you run that example you see that something funny has happened.
This error was probably pretty simple to spot, because we printed out the value of
final_time and it is easy to see that the numbers were just concatenated together rather
than added. So what do we do about the problem? We will need to convert both
int. At this stage of your programming development, it can be a good
idea to include the type of the variable in the variable name itself. So lets look at another
iteration of the program that does that, and the conversion to integer.
Now, that’s a lot better, and in fact depending on the hours you chose, it may be exactly right. If you entered 8 for the current time and 5 for the wait time then 13 is correct. But if you entered 17 (5pm) for the hours and 9 for the wait time then the result of 26 is not correct. This illustrates an important aspect of testing, which is that it is important to test your code on a range of inputs. It is especially important to test your code on boundary conditions. In this case you would want to test your program for hours including 0, 23, and some in between. You would want to test your wait times for 0, and some really large numbers. What about negative numbers? Negative numbers don’t make sense, but since we don’t really have the tools to deal with telling the user when something is wrong we will not worry about that just yet.
So finally we need to account for those numbers that are bigger than 23. For this we will need one final step, using the modulo operator.
Of course even in this simple progression, there are other ways you could have gone astray.
3.2.2. 👩💻 Beginning tips for Debugging¶
Debugging a program is a different way of thinking than writing a program. The process of debugging is much more like being a detective. Here are a few rules to get you thinking about debugging.
Everyone is a suspect (Except Python)! It’s common for beginner programmers to blame Python, but that should be your last resort. Remember that Python has been used to solve CS1 level problems millions of times by millions of other programmers. So, Python is probably not the problem.
Check your assumptions. At this point in your career you are still developing your mental model of how Python does its work. Its natural to think that your code is correct, but with debugging you need to make your code the primary suspect. Even if you think it is right, you should verify that it really is by liberally using print statements to verify that the values of variables really are what you think they should be. You’ll be surprised how often they are not.
Find clues. This is the biggest job of the detective and right now there are two important kinds of clues for you to understand.
3.2.3. Types of Errors¶
Three kinds of errors can occur in a program: syntax errors, runtime errors, and semantic errors. It is useful to distinguish between them in order to track them down more quickly. We will explore these three different types of errors in the next sections of this chapter. Check your understanding