Step 2. Debugging Your First Python Application

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):

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

After editing, we get the following:

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 button, go to the Console to ask for the car's average speed (type 'S'), and we can see that we hit our breakpoint.

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

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

If you click the Step Into button (), 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:

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

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

See the Stepping toolbar and Stepping Through the Program sections for details.

Watching

PyCharm allows you to watch any variable. Just click on the toolbar of the Watches tab, and type the name of the variable you want to watch - let it be my_car.time. Note that code completion is available here:

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

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

See Adding, Editing and Removing 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 hit any breakpoint, PyCharm shows you the value of many of your variables right in the editor:

This inline debugging feature is enabled by default. If you don't see the inline debugging values, please check that it's enabled using the settings icon on the Debug Tool Window:

Evaluating expressions

Finally, you can evaluate any expression at any time. For example, if you want to see the value of a variable, click the button.

Then in the dialog box that opens, click Evaluate:

Actually, you could see the same thing with a watch. With evaluate expression you can do things that you can't do with a watch: you can change things.

For example, if you enter the desired value of the odometer, say, 50, and then continue stepping through your script, you will get the following:

See the Evaluating Expressions section for details.

You can also click the button to enable the Python console in the Debugger. In this mode, you can enter some commands and expressions. For example, you can modify variable values.