1. Debugging Taken from notes by Dr. Neil Moore
CS 115 Lecture
Debugging
Taken from notes by Dr. Neil Moore

2. Debugging
We’ve seen how to write test cases to help you find bugs in your program. What should you do when you find something is wrong?
Find the line that’s wrong and fix it
Which line is wrong?
We know the symptoms, not the disease
So our job is like that of a doctor
Find out what’s making the patient sick!

3. Debugging Sometimes just reading and tracing the program is enough
And once upon a time that was the only option
But doctors can run tests, ask questions…
If we could interact with the program while it’s running… we might be able to see the bug as it happens, we need “X-Ray vision”

4. The debugger
The debugger is a tool for controlling and inspecting a program as it runs
Part of most IDEs (WingIDE and IDLE for sure)
It does not find the bugs for you
Instead, it
“Slows down” the program by letting you run it one line at a time (step through code)
Shows you variables and their values as they change
Shows what functions are currently running (“call stack”)

5. Breakpoints
When using the debugger, you usually start by setting a breakpoint.
This tells the debugger to pause the program just before it executes that line.
In WingIDE, click next to the line number
A red dot appears to show there is a breakpoint there.
Click it again to turn it off
Note: breakpoints are not saved with the program!
In IDLE, right click the line of code and choose “Set breakpoint”.
The line will be highlighted in yellow

6. Breakpoints
To run with the debugger in WingIDE, click the “Debug” icon, the program will start running
In IDLE select Debug → Debugger then run the program normally (using Run Module)
The program will run full-speed until control reaches the breakpoint
Then the program pauses, the IDE gives you control
If you run the program without using “Debug”, breakpoints are ignored, the program runs normally.
You may have to put in more than one breakpoint if you have branches or loops, to make sure execution actually reaches one of them

7. Single-stepping
Single stepping means running the next line of the program. In most IDEs there are 3 ways to single-step.
Step Over (“Over” in IDLE)
Execute the current line, pausing again when it finishes
If the line calls a function, the whole function’s code will be executed without being seen

8. Single-stepping Step Into (“Step” in IDLE) Step Out (“Out” in IDLE)
Execute the current line, pausing at the start of the next line to be executed
If the line calls a function, execution will pause at the start of that function’s definition
Step Out (“Out” in IDLE)
Executing the rest of this function, pausing after it returns
Stepped into a function you didn’t want to see? Use this to get back to the calling code

9. Single-stepping
More about the difference in the different modes when we cover functions
These stepping functions only work when your program is paused by the debugger
So only in debug mode
And you must have a breakpoint set somewhere

10. Debugging variables
The watch window shows you the variables that exist before executing the current line
In WingIDE, called “Stack Data”
In IDLE, called “Locals”
Shows variables that are in scope
Already defined in this function
Not destroyed yet (at end of the function)
Also shows their values and types
Watch this window when single-stepping
That will show you which variables are changing and how
Make sure they change as you expect them to!