Demonstrating a programmer-defined exception class. : Error Exception « Statement « Python Tutorial

1. Python Tutorial
2. Statement
3. Error Exception

import math

class NegativeNumberError( ArithmeticError ):
   """Attempted improper operation on negative number."""
   pass

def squareRoot( number ):
   """Computes square root of number. Raises NegativeNumberError
   if number is less than 0."""

   if number < 0:
      raise NegativeNumberError, "Square root of negative number not permitted"

   return math.sqrt( number )

while 1:
   try:
      userValue = float( raw_input( "\nPlease enter a number: " ) )
      print squareRoot( userValue )
   except ValueError:
      print "The entered value is not a number"
   except NegativeNumberError, exception:
      print exception
   else:
      break