1. Debuggers and Debugging
Professor Hugh C. Lauer CS-1004 — Introduction to Programming for Non-Majors
(Slides include materials from Python Programming: An Introduction to Computer Science, 2nd edition, by John Zelle and copyright notes by Prof. George Heineman of Worcester Polytechnic Institute)
CS-1004, A-Term 2014
Debuggers and Debugging

2. Debuggers and Debugging
A program that “runs” another program so that
You can cause it to stop or pause at designated places
Examine the values of variables when stopped
See where it has been called from
Step one statement at a time
Step over entire functions …
Essential to program development in all modern languages
It is too difficult …
… for ordinary humans …
… to develop non-trivial programs
… without the aid of a debugger
CS-1004, A-Term 2014
Debuggers and Debugging

3. Debuggers and Debugging
Breakpoint
A “flag” inserted into code to cause running program to suspend just before executing that statement
Return control to debugger
In Python:–
Interpreter notices flag
Calls debugger
In compiled languages
Debugger inserts special machine instruction in compiled code
Triggers “interrupt”
Interrupt-handler in OS transfers control to debugger
Debugger may resume execution of running program
As if nothing had happened (except passage of time)
CS-1004, A-Term 2014
Debuggers and Debugging

4. Breakpoint (continued)
Debugger “knows” the source code
i.e., which line is currently executing
which line will execute next
where variables are stored
what variables are currently being changed
In Python, because it is interpreted language
In compiled languages, compiler plants extra info in machine code to connect to source code Go
Debugger command to program to run normally until a breakpoint
CS-1004, A-Term 2014
Debuggers and Debugging

5. Debuggers and Debugging
Single-step
Debugger places flag in interpreter or processor to suspend execution at every statement
Python interpreter “knows” whenever it is starting a new statement
E.g., next statement in-line
Branch of if-else
Loop iteration …
In compiled languages,
Special modes in processor
CS-1004, A-Term 2014
Debuggers and Debugging

6. Single-step scenarios
Simple Python statements
Stops at start of every statement
Function call, or
Expression containing function call
Stops at first statement inside function
Function return
Stops at first statement after function call
(including first statement inside another function)
(not including system & library functions)
CS-1004, A-Term 2014
Debuggers and Debugging

7. Debuggers and Debugging
Clear Breakpoint
Removes breakpoint
Handy when you are satisfied with a piece of code and want to go on to debug next piece
CS-1004, A-Term 2014
Debuggers and Debugging

8. Debuggers and Debugging
Questions?
CS-1004, A-Term 2014
Debuggers and Debugging

9. Debuggers and Debugging
Step Over
Debugger places (temporary) breakpoint at start of next statement
Or function call, etc.
Runs freely until next breakpoint
(Temporary) breakpoint placed above
Any other breakpoint
CS-1004, A-Term 2014
Debuggers and Debugging

10. Debuggers and Debugging
Step Out
Find caller of current function
Place (temporary) breakpoint immediately after that call
At start of next statement
CS-1004, A-Term 2014
Debuggers and Debugging

11. Debuggers and Debugging
Viewing variables
IDLE debugger lists all variables in current scope of current function
Click on call history to see variables in scope of calling function(s)
Can maneuver up and down ‘call stack’
Check source box to track execution
i.e., highlight each line as it is executing
Check the box to show globals
I.e., variables, etc., defined outside of any function
CS-1004, A-Term 2014
Debuggers and Debugging

12. Debuggers and Debugging
Questions?
CS-1004, A-Term 2014
Debuggers and Debugging