CSC111 Lab 9 2015

--D. Thiebaut (talk) 07:14, 29 March 2015 (EDT)

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Exceptions

preparation

• Create a new program called lab9_1.py, and copy this code to the new Idle window.

# lab9_1.py
# Your name here

# getInput: returns an integer larger
# than 0.  Expected to be robust
def getInput():
   
   while  True:
      x = int( input( "Enter an integer greater than 0: " ) )
      if x <= 0:
         print( "Invalid entry.  Try again!" )
      else:   
         return x

def main():
   num = getInput()
   print( "You have entered", num )

main()

• Test it with numbers such as -3, -10, 0, 5. Verify that the input function works well when you enter numbers.
• Test your program again, and this time enter expressions such as "6.3", or "hello" (without the quotes).
• Make a note of the Error reported by Python:

• Modify your function and add the highlighted code below by hand. This code will catch the ValueError exception.

# getInput: returns an integer larger
# than 0.  Catches Value errors
def getInput():
    
   # repeat forever...
   while  True:

      # try to get an int
      try:
          x = int( input( "Enter an integer greater than 0: " ) )
      except ValueError:
          # the user must have entered something other than an int
          print( "Invalid entry.  Not an integer.  Try again!" )
          continue

      # No errors caught.  See if the number is negative
      if x <= 0:
         print( "You entered a negative number.  Try again!" )
      else:   
         # finally, we can return x as it is an int that is >0
         return x

• Run your program and try different invalid inputs, such as strings or floats. You can also try just pressing the Return key, indicating that you are not providing anything to the input function. Verify that your program catches all these invalid entries and does not crash.

Exercise

• 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()