Python exceptions are a safety net which let you catch data or program problems or other unexpected issues without having to explicity code each and every potential problem or problem group - a wonderful fail safe mechanism.

If you fail to write your Python code with try and except blocks, it will use the default (built in) exception handler, which give you a full stack trace (a confession) and exits the program. Thus

  def doit():
    a = 6 /0
  doit()

gives

  wizard:nov12 graham$ python tiny
  Traceback (most recent call last):
    File "tiny", line 3, in
      doit()
    File "tiny", line 2, in doit
      a = 6 /0
  ZeroDivisionError: integer division or modulo by zero
  wizard:nov12 graham$

Within your own exception handlers ...
• you can pick up your exception object and generate your own message.
• you can use the message and __class__.__name__ objects to report Python's own message and class name
• you can use the traceback module to print out a full confession

Here's a code snippet showing you the standard and traceback items in use:
  except Exception,e:
    print e.message
    print e.__class__.__name__
    traceback.print_exc(e)

A full example including traceback use from yesterday's Intermediate Python Course is [here].
(written 2012-11-22, updated 2012-11-24)

 Y109 - Python - Exceptions
  [381] Exceptions in Python - (2005-07-17)
  [1042] Nested exceptions in Python - (2007-01-18)
  [1236] Trying things in Python - (2007-06-18)
  [2018] UnboundLocalError - Python Message - (2009-01-31)
  [2281] Python - using exceptions to set a fallback - (2009-07-12)
  [2368] Python - fresh examples of all the fundamentals - (2009-08-20)
  [2408] Robust user input (exception handling) example in Python - (2009-09-17)
  [2622] Handling unusual and error conditions - exceptions - (2010-02-03)
  [2994] Python - some common questions answered in code examples - (2010-10-10)
  [2998] Using an exception to initialise a static variable in a Python function / method - (2010-10-13)
  [3177] Insurance against any errors - Volcanoes and Python - (2011-02-19)
  [3433] Exceptions - a fail-safe way of trapping things that may go wrong - (2011-09-11)
  [3441] Pressing ^C in a Python program. Also Progress Bar. - (2011-09-15)
  [3664] Error checking in a Python program - making your program robust via exceptions - (2012-03-22)
  [3913] How many times ... has this loco headed west through Tenby? - Python exceptions - (2012-11-05)
  [4029] Exception, Lambda, Generator, Slice, Dict - examples in one Python program - (2013-03-04)
  [4161] Python varables - checking existance, and call by name or by value? - (2013-08-27)
  [4444] Elements of an exception in Python - try, except, else, finally - (2015-02-28)

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