CSC111 Exercises with Exceptions: try/except

--D. Thiebaut (talk) 09:05, 2 April 2014 (EDT)

---

The exercises below deal with protecting Python code that might crash with try/except statements to make the program robust.

Code in need of try/except statements

• The program below is not very robust. We can easily make it crash.
• Observe each function and see how to make it fail, or see why it will fail the way it is called
• Make the program crash. Register the XXXXError that is generated. For example, if the output of the crash looks like this:

Traceback (most recent call last):
 File "/Users/thiebaut/Desktop/except0.py", line 29, in <module>
   main()
 File "/Users/thiebaut/Desktop/except0.py", line 27, in main
   example3( [ 10, 3, 5, 6 ] )
 File "/Users/thiebaut/Desktop/except0.py", line 18, in example3
   sum = sum + L[i]
IndexError: list index out of range

what you are interested in is IndexError. This is the exception you want to guard your code against.

  try:
      ........
      ........
  except IndexError:
      .........

• Verify that you have made your functions more robust to erroneous input/data.

Code

def example1():
    for i in range( 3 ):
        x = int( input( "enter a number: " ) )
        y = int( input( "enter another number: " ) )
        print( x, '/', y, '=', x/y )

def example2( L ):
    print( "\n\nExample 2" )
    sum = 0
    sumOfPairs = []
    for i in range( len( L ) ):
        sumOfPairs.append( L[i]+L[i+1] )

    print( "sumOfPairs = ", sumOfPairs )


def printUpperFile( fileName ):
   file = open( fileName, "r" )
   for line in file:
       print( line.upper() )
   file.close()
    
def main():
    example1()
    L = [ 10, 3, 5, 6, 9, 3 ]
    example2( L )
    example2( [ 10, 3, 5, 6, "NA", 3 ] )
    example3( [ 10, 3, 5, 6 ] )

    printUpperFile( "doesNotExistYest.txt" )
    printUpperFile( "./Dessssktop/misspelled.txt" )

main()

Solution

def example1():
    while True:
       try:
            x = int( input( "enter a number: " ) )
            y = int( input( "enter another number: " ) )
            print( x, '/', y, '=', x/y )
            break
       except ZeroDivisionError:
            print( "Can't divide by 0!" )
       except ValueError:
            print( "That doesn't look like a number!" )
       except:
            print( "something unexpected happend!" )
            
            
def example2( L ):
    print( "\n\nExample 2" )
    print( "L          = ", L )
    sum = 0
    sumOfPairs = []
    for i in range( len( L ) ):
            try:
                sumOfPairs.append( L[i]+L[i+1] )
            except IndexError:
                continue
            except TypeError:
                continue
    
    print( "sumOfPairs = ", sumOfPairs )

def printUpperFile( fileName ):
    try:
       file = open( fileName, "r" )
    except FileNotFoundError:
       print( "***Error*** File", fileName, "not found!" )
       return False
    
    for line in file:
        print( line.upper() )
    file.close()
    return True
    
def main():
    example1()
    
    L = [ 10, 3, 5, 6, 9, 3 ]
    example2( L )

    L = [ 10, 3, "NA", 6, 9, 3 ]
    example2( L )
    
    open( "doesNotExistYest.txt", "w" ).close()

    printUpperFile( "doesNotExistYest.txt" ):
        
    printUpperFile( "./Dessssktop/misspelled.txt" )


main()