CSC111 Lab 10 2014

--D. Thiebaut (talk) 13:39, 7 April 2014 (EDT)

---

Exceptions

If you feel you have a good handle on exceptions, then move on to the next section. Otherwise do the exercises of this section.

TypeError Exceptions

The code section below should remind you of a recent homework assignment. Create a program called stats.py with this code:

statistics = [['Afghanistan', 28710000, 'NA', 'NA', 1], ['Albania', 3580000,
               12000, 'NA', 10], ['Algeria', 32810000, 180000, 'NA', 2],
               ['Andorra', 69150, 24500, 'NA', 1], ['Angola', 10760000,
               60000, 'NA', 1], ['Anguilla', 12738, 919, 'NA', 16],
               ['Antigua_and_Barbuda', 67897, 5000, 'NA', 16], ['Argentina',
               38740000, 4650000, 'NA', 33], ['Armenia', 3320000, 30000, 'NA',
               9], ['Aruba', 70844, 24000, 'NA', 'NA'], ['Australia', 19730000,
               13010000, 9020000, 571], ['Austria', 8180000, 4650000,
               1300000, 37], ['Azerbaijan', 7830000, 25000, 'NA', 2],
               ['Bahrain', 667238, 140200, 'NA', 1], ['Bangladesh',
               138440000, 150000, 'NA', 10], ['Barbados', 277264, 6000, 'NA',
               19], ['Belarus', 10330000, 422000, 'NA', 23], ['Belgium',
               10280000, 4870000, 1600000, 61], ['Belize', 266440, 18000,
               'NA', 2], ['Benin', 7040000, 25000, 'NA', 4], ['Bhutan',
               2130000, 2500, 'NA', 'NA'], ['Bolivia', 8580000, 78000, 'NA', 9],
               ['Bosnia_and_Herzegovian', 3980000, 45000, 'NA', 3],
               ['Botswana', 1570000, 33000, 'NA', 11], ['Brazil', 182030000,
               22320000, 10860000, 50], ['Brunei', 358098, 35000, 'NA', 2],
               ['Bulgaria', 7530000, 1610000, 'NA', 200], ['Burkina_Faso',
               13220000, 25000, 'NA', 1], ['Burma', 42510000, 10000, 'NA', 1],
               ['Burundi', 6090000, 6000, 'NA', 1], ['Cambodia', 13120000,
               10000, 'NA', 2], ['Cameroon', 15740000, 45000, 'NA', 1] ]

def main():
    for country, pop, users, active, isp in statistics:
        print( "%30s (%d habitants)" % (country, pop ), end="" )
        print( "Users=", users, " Active=", active, " ISP=", isp )


main()

• Run the program.
• Notice that the data are in their original form. When a quantity is not known, the creators of the list use "NA" to indicate that it was Not Available.

• Let's compute the total population of the (partial) list of countries in the list statistics.

    count = 0
    for country, pop, users, active, isp in statistics:
         count += pop

Go ahead and see if your get a total population of 618039669.

Challenge #1

• Modify your program so that it also computes the total number of Users. Make it use the very same construct as shown above, but now for the quantity users.

• Run your program.

• Did you get an error of this type?

Traceback (most recent call last):
  File "stats.py", line 36, in <module>
        main()
  File "stats.py", line 31, in main
        userCount += users
TypeError: unsupported operand type(s) for +=: 'int' and 'str'

If so, why? What made the program crash?

Think hard!

Harder!

Do not move on until you know for sure why this error was generated.

• I trust that if you are reading this, it is because you know what caused your program to crash. The clue is unsupported operand type(s) for +=: 'int' and 'str' , indicating that we are trying to add 'NA' which is a string to count, which is an integer.

We could use an if statement to test whether users contains 'NA' or not, but we'll use another approach. We will tell python to try adding users to the counting variable, and if there is an error of type TypeError (see the last ligne of the error message when the program crashed), then we won't add anything to the count variable; we'll just pass.

The main program becomes:

 def main():
     popCount = 0
     userCount = 0
     for country, pop, users, active, isp in statistics:
         print "%30s (%d habitants)" % (country, pop ),
         print "Users=", users, " Active=", active, " ISP=", isp
         popCount += pop
         try:
             userCount += users
         except TypeError:
             # don't do anything
             pass 
     print "total population = ", popCount
     print "total users      = ", userCount

• Try it!
• Use the same method to compute the total number of ISPs.

Other Exceptions

• Imagine that now the list statistics is defined as shown below, and that for some countries the ISP value (the last one) is missing:

 
statistics = [['Afghanistan', 28710000, 'NA', 'NA', 1], ['Albania', 3580000,
                12000, 'NA', 10], ['Algeria', 32810000, 180000, 'NA'],
                ['Andorra', 69150, 24500, 'NA', 1], ['Angola', 10760000,
                60000, 'NA', 1], ['Anguilla', 12738, 919, 'NA', 16],
                ['Antigua_and_Barbuda', 67897, 5000, 'NA', 16], ['Argentina',
                38740000, 4650000, 'NA', 33], ['Armenia', 3320000, 30000, 'NA',
                9], ['Aruba', 70844, 24000, 'NA' ], ['Australia', 19730000,
                13010000, 9020000, 571], ['Austria', 8180000, 4650000,
                1300000, 37], ['Azerbaijan', 7830000, 25000, 'NA'],
                ['Bahrain', 667238, 140200, 'NA', 1], ['Bangladesh',
                138440000, 150000, 'NA', 10], ['Barbados', 277264, 6000, 'NA',
                19], ['Belarus', 10330000, 422000, 'NA', 23], ['Belgium',
                10280000, 4870000, 1600000, 61], ['Belize', 266440, 18000,
                'NA', 2], ['Benin', 7040000, 25000, 'NA', 4], ['Bhutan',
                2130000, 2500, 'NA', 'NA'], ['Bolivia', 8580000, 78000, 'NA', 9],
                ['Bosnia_and_Herzegovian', 3980000, 45000, 'NA', 3],
                ['Botswana', 1570000, 33000, 'NA', 11], ['Brazil', 182030000,
                22320000, 10860000, 50], ['Brunei', 358098, 35000, 'NA', 2],
                ['Bulgaria', 7530000, 1610000, 'NA', 200], ['Burkina_Faso', 
                13220000, 25000, 'NA'], ['Burma', 42510000, 10000, 'NA', 1],
                ['Burundi', 6090000, 6000, 'NA', 1], ['Cambodia', 13120000,
               10000, 'NA', 2], ['Cameroon', 15740000, 45000, 'NA', 1] ]

• Write a program (or copy/paste part of the previous one) that will read the list above, and output the total population (which should be the same as before).
  • For this you should write your program as if all the country lists contained 5 quantities.
  • Then you run your program.
  • You observe that it crashes when it tries to extract the 5th quantity from a country list.
  • You note the name of the error generated (which is on the last line, and usually of the form XxxxError, such as IndexError, TypeError, NameError, ValueError, etc.)
  • Add a try/except statement around the code that generates the error (Python refers to a run-time error that creates a crash as an exception):

      try:
          country, pop, users, active, isp = someVariableOfYours
      except XxxxError:
          country, pop, users, active = someVariableOfYours

Run your program again and observe that it doesn't crash and that it reports the correct information.