CSC111 Exercises with Exceptions: try/except

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

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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():
    x = eval( input( "enter a number: " ) )
    y = eval( input( "enter another number: " ) )
    print( x, '/', y, '=', x/y )

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

    print( "sum = ", sum )

def example3( L ):
    print( "\n\nExample 3" )
    sum = 0
    for i in range( 6 ):
        sum = sum + L[i]

    print( "sum = ", sum )
    
def main():
    example1()
    L = [ 10, 3, 5, 6, 9, 3 ]
    example2( L )
    example2( [ 10, 3, 5, 6, "NA", 3 ] )
    example3( [ 10, 3, 5, 6 ] )
    
main()

File Examples

Example 1

What could possibly go wrong with this code?

def printUpperFile( fileName ):
   file = open( fileName, "r" )
   for line in file:
       print( line.upper() )
   file.close()

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

...