Exceptions and pre-conditions

From Computer Science Wiki

An exception is an anomalous or exceptional condition requiring special processing – often changing the normal flow of program execution[2].

Be careful not to confuse an exception with an error.

An error "indicates serious problems that a reasonable application should not try to catch." An exception "indicates conditions that a reasonable application might want to catch."[3].

We use the term "catch an exception" to catch exceptions and plan how the program should respond.


# here is some Python code with a deliberate error. There should be two apostrophe's for the end argument.
print("hello world", end=')
print("from Warsaw")
print("=" * 25)

When we execute this code we see the following exception:

Exception example.png

You can see there are four lines here:

  1. on the first line we see the name of the file being executed and the line number where the exception was thrown.
  2. on the second line we see the line of code where the exception was thrown
  3. on the third line we see a carat (^) pointing the place where the exception was thrown
  4. on the fourth line we see the description of the exception

If you google the exception (line 4) you can often find helpful information about the exception. For a list of common exceptions, please click here


These standards are used from the IB Computer Science Subject Guide[4]

  • Identify exceptions that need to be considered in a specified problem solution.


  1. http://www.flaticon.com/
  2. https://en.wikipedia.org/wiki/Exception_handling
  3. https://stackoverflow.com/questions/5813614/what-is-difference-between-errors-and-exceptions
  4. IB Diploma Programme Computer science guide (first examinations 2014). Cardiff, Wales, United Kingdom: International Baccalaureate Organization. January 2012.