Carnegie Mellon Worcester Polytechnic Institute Debuggers and Debugging

Carnegie Mellon Worcester Polytechnic Institute 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, 2 nd edition, by John Zelle and copyright notes by Prof. George Heineman of Worcester Polytechnic Institute) CS-1004, A-Term 2014 Debuggers and Debugging 1

Carnegie Mellon Worcester Polytechnic Institute Debugger ¢ ¢ ¢ 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 … ¢ ¢ ¢ CS-1004, A-Term 2014 … for ordinary humans … … to develop non-trivial programs … without the aid of a debugger Debuggers and Debugging 2

Carnegie Mellon Worcester Polytechnic Institute 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 3

Carnegie Mellon Worcester Polytechnic Institute 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 4

Carnegie Mellon Worcester Polytechnic Institute 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 5

Carnegie Mellon Worcester Polytechnic Institute 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 6

Carnegie Mellon Worcester Polytechnic Institute 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 7

Carnegie Mellon Worcester Polytechnic Institute Questions? CS-1004, A-Term 2014 Debuggers and Debugging 8

Carnegie Mellon Worcester Polytechnic Institute 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 9

Carnegie Mellon Worcester Polytechnic Institute 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 10

Carnegie Mellon Worcester Polytechnic Institute 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 11

Carnegie Mellon Worcester Polytechnic Institute Questions? CS-1004, A-Term 2014 Debuggers and Debugging 12