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

1. Required Reading
2. Handling Exceptions (try/except)
3. The Stack Trace
4. Raising Exceptions (raise)
5. When not to raise exceptions
6. Clean-up Actions (finally)
7. Predefined Clean-up Actions (with)
8. Built-In Exceptions and the Exception Hierarchy

1. Required Reading
   1. The Python Tutorial, Chapter 8 (Errors and Exceptions), except 8.5
   2. The Python Standard Library, Chapter 6 (Built-In Exceptions)
       
2. Handling Exceptions (try/except)

   #1) Simplest form
   
   def isFactor(factor, n):
       return (n % factor == 0)
   try:
       if isFactor(0, 2): print "0 is a factor of 2"
       if isFactor(2, 0): print "2 is a factor of 0"
   except:
       print "We just caught an error"
   
   #2) With Exception information
   
   try:
       if isFactor(0, 2): print "0 is a factor of 2"
       if isFactor(2, 0): print "2 is a factor of 0"
   except Exception as error:
       print "We just caught this error:", error
   
   #3) Catching specific exception types
       
   try:
       prompt = "Enter a number to invert: "
       n = float(raw_input(prompt)) # non-float ==> ValueError
       inverse = 1/n                # n==0 ==> ZeroDivisionError
       if (n == 42): ruhRoh()       # no such function!
       print "The inverse of", n, "is", inverse
   except ZeroDivisionError:
       print "Cannot divide by 0."
   except ValueError:
       print "You did not enter a number!"
   except Exception as error:
       print "Unknown error:", error
       print "   Error type:", type(error)

3. The Stack Trace

   #1) Buggy code that crashes (look at stack trace for clues)
   
   # intentionally buggy code here!
   def isFactor(factor, n):
       return (n % factor == 0)
   
   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 isFactor(n, digit):
               #found another digit that divides n
               count += 1
       return count
   
   #8 is a factor of 80, but 0 is not, so digitFactors(80) should return 1
   print digitFactors(80) #crashes
   
   #2) Once again, but catching the exception to print out more information
   #  (but we lost the stack trace!)
   
   def isFactor(factor, n):
       return (n % factor == 0)
   
   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
           try:
               if isFactor(n, digit):
                   #found another digit that divides n
                   count += 1
           except:
               print "crashed on this input:", n, digit
       return count
   
   print digitFactors(80) #does not crash (exception quietly caught)
   
   #3) Yet again, this time explicitly printing the stack trace, and not crashing!
   
   import traceback, sys
   
   def isFactor(factor, n):
       return (n % factor == 0)
   
   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
           try:
               if isFactor(n, digit):
                   #found another digit that divides n
                   count += 1
           except Exception as error:
               print "crashed on this input:", n, digit
               print "Error:", error
               traceback.print_exc(file=sys.stdout)
       return count
   
   print digitFactors(80) #does not crash, but prints stack trace!
   
   #4) Once more, this time re-raising the exception, so yet again crashing
   
   def isFactor(factor, n):
       return (n % factor == 0)
   
   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
           try:
               if isFactor(n, digit):
                   #found another digit that divides n
                   count += 1
           except:
               print "crashed on this input:", n, digit
               raise
       return count
   
   print digitFactors(80) #crashes!

4. Raising Exceptions (raise)

   def f(n):
       if (n == 42):
           raise Exception("I take exception to 42.")
       return n+1
   
   try:
       print f(5)
       print f(42)
       print f(50)
   except Exception as error:
       print "Caught exception:", error

5. When not to raise exceptions

   #1) Unnecessary raise
   
   def isFactor(factor, n):
       if (factor == 0):
           raise Exception("Cannot divide by 0")  #wrong!
       return (n % factor == 0)
   
   print isFactor(0, 8)
   
   #2) Here's why...
   
   def isFactor(factor, n):
       return (n % factor == 0)
   
   print isFactor(0, 8)
   
   #3) But you may use assert here, if this violates a pre-condition
   #   (which it presumably does)
   
   def isFactor(factor, n):
       assert(factor != 0)
       return (n % factor == 0)
   
   print isFactor(0, 8)

6. Clean-Up Actions (finally)

   #1) Wrong way
   
   filename = "non-existent-file.txt"
   try:
       file = open(filename, "r")
       print filename, "has", len(file.readlines()), "lines"
   except IOError:
       print "Cannot open file", filename
   finally:
       file.close()  #error: file not defined here!
   
   #2) Right way
   
   filename = "non-existent-file.txt"
   file = None
   try:
       file = open(filename, "r")
       print filename, "has", len(file.readlines()), "lines"
   except IOError:
       print "Cannot open file", filename
   finally:
       if (file != None): file.close()  #ahh, that's better

7. Predefined Clean-up Actions (with)

   filename = "non-existent-file.txt"
   try:
       # using "with", no need for file.close() in finally clause
       with open(filename, "r") as file:
           print filename, "has", len(file.readlines()), "lines"
   except IOError:
       print "Cannot open file", filename

8. Built-In Exception and the Exception Hierarchy
   1. Exception Descriptions
   2. The Exception Hierarchy

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