Step 2. Debug your first Python application

Last modified: 17 March 2022

Finding out the origin of the problem

Remember, in the previous tutorial you've created and run the Car script? Let’s play a little more with it and modify the average_speed function as follows:

def average_speed(self):
    return self.odometer / self.time

Let's see what happens when we start our script up, and try to find out our average speed:

Error messages

Oops... PyCharm reports a runtime error: a ZeroDivisionError.

Let’s dig a little deeper into our code to find out what’s going wrong. We can use the PyCharm debugger to see exactly what’s happening in our code. To start debugging, you have to set some breakpoints first. To create breakpoints, just click in the gutter

Adding breakpoints

Next, click the Run icon icon in the gutter, next to the main clause, and choose Debug 'Car'.

Debug command

PyCharm starts a debugging session and shows the Debug tool window

Debug tool window

Click the Resume icon button to proceed with the script execution and in the Console tab, enter S and press Enter:

Debug tool window: Console tab

Click the Resume icon button to resume the script execution. The exception is here. Another breakpoint appeared as well: by default PyCharm will halt for any exception that wasn't caught in your code, and it'll show an icon of a breakpoint with a lightning bolt.

Exception breakpoint

The debugger also shows the error message. So we’ve found our problem. You can also see in the debugger, that the value self.time is equal to zero:

aself.time is equal to zero

Surrounding code

To avoid running into the same problem again, let's add an if statement to check whether the time equals zero. To do that, select the statement return self.odometer / self.time in the method average_speed and then press Ctrl+Alt+T (Code | Surround with):

Surround code

PyCharm creates a stub if construct, leaving you with the task of filling it with the proper contents.

A stub for surrunding with an if statement

After editing, we get the following:

Results of the surrounded code

Let's take a closer look to see how the debugger can show your what your code is doing.

Debugging in detail

The Debug tool window shows dedicated panes for frames, variables, and watches, and the console, where all the input and output information is displayed. If you want the console to be always visible, you can drag it to one of the PyCharm window's edges.

Stepping

If you want to see what your code does line by line, there's no need to put a breakpoint on every line, you can step through your code.

Let's see what it looks like to step through our example program: click the Resume icon button, go to the Console to ask for the car's average speed (type 'S'), and we can see that we press our breakpoint.

We can use the stepping toolbar buttons to choose on which line we'd like to stop next.

Stepping toolbar

For example, click the Step Over button Step over icon and see the blue marker moving to the next line of code:

Stepping over during the debugging

If you click the Step Into button Step into icon, you will see that after the line action = input("What should I do? [A]ccelerate, [B]rake, " "show [O]dometer, or show average [S]peed?").upper() the debugger goes into the file parse.py:

Stepping into during the debugging

However, if you continue using Step over icon, you'll see that your application just passes to the next loop:

Debugging: passing to the next loop

If you want to concentrate on your own code, use the Step Into My Code button Step into my code - thus you'll avoid stepping into library classes.

See the Stepping toolbar and Step through the program sections for details.

Watching

PyCharm allows you to watch any variable. Just type the name of the variable you want to watch in the Evaluate and Watch field - let it be my_car.time. Note that code completion is available here. Then click The Add button on next to the field.

Watch completion

At first, you see the time equals nil - it means that the variable is not yet defined:

Watch error

However, when the program execution continues to the scope that defines the variable, the watch gets the following view:

Watched variable gets a value

See Watches section for details.

Inline debugging

You may have noticed another PyCharm feature that makes it easy to see what your code is doing: the inline debugger. As soon as you press any breakpoint, PyCharm shows you the value of many of your variables right in the editor:

Inline debugging

This inline debugging feature is enabled by default. If you don't see the inline debugging values, check that it's enabled using the settings icon Settings on the debug toolbar :

Show values inline

Summary

So, you've done it! Congrats! Let's repeat what you've done with the help of PyCharm:

  • Found out the origin of the problem

  • Set breakpoints

  • Stepped through your program

  • Created a watch

  • Evaluated an expression

See also

Procedures

Tutorials and Examples