Computer Science 15-112, Fall 2011
Class Notes:  Debugging

1. Required Reading
2. Stack Traces -- A Sea of Information!
3. Debug with print Statements
4. Actually, use db() instead of print
5. Use Assertions and Invariants
6. Disable Events + Simulate Events
7. Profile with time.time() and cProfile.run("myTest()")
8. Use the pdb Python debugger

1. Required Reading
   1. Python Standard Library, Ch 26.2 - 26.3 (pdb: The Python Debugger + Debugger Commands)
       
2. Stack Traces -- A Sea of Information!
    
3. Debug with print Statements

   # sample buggy code
   def digitFactors(n):
       # returns the number of digits of n that are factors of n
       count = 0
       while (n > 0):
           digit = n % 10
           n /= 10
           if (n % digit == 0):
               # found another digit that divides n
               count += 1
       return count
   
   # 6367 is prime, so digitFactors(6367) should equal 0
   print digitFactors(6367) # prints 2 (uh oh)

4. Actually, use db() instead of print
   1. Sample Code

      def debug():
          return __debug__  # set to False before you submit your hw!
      
      def db(*args):
          if debug():
              for arg in args:
                  print arg,
              print
      
      x = 0
      for n in xrange(5):
          x += n
          db("n=",n,", x=",x)
      print x

   2. debug() vs __debug__
       
5. Use Assertions and Invariants
   See "Correctness" notes
    
6. Disable Events + Simulate Events
   (This applies once we cover graphics and events, later in the course)
    
7. Profile with time.time() and cProfile.run("myTest()")
    
8. Use the pdb Python debugger
   • break into debugger with: import pdb; pdb.set_trace()
   • some commands:

     h	help
     l	list source code near the current line
     w	where (print stack trace)
     a	arglist: print the arglist of the current function
     p expr	print the value of the expression
     pp expr	same as p only "pretty-printed"
     s	step into called functions
     n	next (step over called functions)
     r	return: run until this function returns
     c	continue (run again)
     !stmt	execute the stmt in the current context
     q	quit from the debugger

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