Computer Science 15-110, Spring 2011
Class Notes:  Debugging

Debugging

• Avoid bugs:
• top-down design
• one task per function
• test functions (also one per function)
• aggressively test as you go
• avoid copy-paste-edit code (bug propagation)
• use good style
  
  
• Find bugs:
• robust test cases
• test every logical path through the code
• test every edge case (maximum, minimum, negatives, 0, positives, first, last, None, ...)
• avoid duplicate/unnecessary test cases (extra work / false sense of security)
• read error message carefully!
• study reported line and previous lines
• study stack trace
• confirm bug is in code and not test case!
• construct minimal error-generating case
• predict (hand tracing) and confirm output
• print statements
  print "x=",x
• conditional print statements
  if (i == 50):
      print "x=",x
• conditional with DEBUG local variable
  DEBUG = True
  ...
  if (DEBUG and (i == 50)):
      print "x=",x
• db function
  def db(msg):
      DEBUG = True
      if (DEBUG):
          print "DEBUG: " + msg
  ...
  db("x=" + str(x))
  if (i == 50):  db("x=" + str(x))
• assertions (type-checking, preconditions, invariants, postconditions)
  def sqrt(x):
      assert(type(x) in [int, float])  # type-checking assertion
      assert(x >= 0)  # precondition assertion
      return x**0.5
• random.seed(x)
• divide-and-conquer (binary-like search through thread of control)
• simulate block comments with """ 
• caution:  """ do not nest
• performance testing with large test cases
  

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