text: #!/usr/bin/env python3
  # Copyright (c) 2008-11 Qtrac Ltd. All rights reserved.
  # This program or module is free software: you can redistribute it and/or
  # modify it under the terms of the GNU General Public License as published
  # by the Free Software Foundation, either version 3 of the License, or
  # (at your option) any later version. It is provided for educational
  # purposes and is distributed in the hope that it will be useful, but
  # WITHOUT ANY WARRANTY; without even the implied warranty of
  # MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
  # General Public License for more details.
  
  class Const:
  
      """
      >>> const = Const()
      >>> const.text = "verified"
      >>> const.limit = 591
      >>> const.text, const.limit
      ('verified', 591)
      >>> const.limit -= 12
      Traceback (most recent call last):
      ...
      ValueError: cannot change a const attribute
      >>> const.x
      Traceback (most recent call last):
      ...
      AttributeError: 'Const' object has no attribute 'x'
      >>> del const.text
      Traceback (most recent call last):
      ...
      ValueError: cannot delete a const attribute
      >>> del const.x
      Traceback (most recent call last):
      ...
      AttributeError: 'Const' object has no attribute 'x'
      """
  
      def __setattr__(self, name, value):
          if name in self.__dict__:
              raise ValueError("cannot change a const attribute")
          self.__dict__[name] = value
  
  
      def __delattr__(self, name):
          if name in self.__dict__:
              raise ValueError("cannot delete a const attribute")
          raise AttributeError("'{0}' object has no attribute '{1}'"
                               .format(self.__class__.__name__, name))
  
  
  if __name__ == "__main__":
      import doctest
      doctest.testmod()
  