COMP 1000: Think Like a Computer: The PreTeXt Interactive Edition

In this textbook, we provide the CodeLens tool so that you are able to step through the program and watch what happens each time a line is evaluated by the Python interpreter. What if you didn’t have CodeLens though, what would you do?

In that case, print statements are your best friend. They can show you what the value of something is. This is especially helpful in the case of writing for loops or accumulating a value. If something is going wrong you can compare what you expect to happen to what is actually happening. Consider the following example, which only prints out the value of the accumulator after the for loop.

Say we weren’t sure what num was being assigned each time we iterated. One way to find out would be to add a print statement inside of the for loop.

If we wanted to see what was happening to tot as well, we could print that out in the for loop too!

Finally, if we wanted to make these numbers easier to understand, we could add more to the print statements to make them easier to read! This is especially important when you have a loop that is complex or will iterate many times. If your console is full of information from print statements, it can be hard to decipher. But adding informative print statements, as in the example below, makes it much easier to see what is happening.

In the above example, we have done two different things. We have added text inside the print statements on lines 7, 9 & 11, to explain what variable is being displayed. We have also added navigational print statements on lines 4, 6, & 10 that don’t print out any variables, but just provide information about where in the code the Python interpreter is executing. This shows us the flow of execution, and helps provide context for the variable print statements. You won’t always need this many print statements, but if your code is not behaving as you expect and you are really struggling to identify the problem, these types of print statements will be very helpful.