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Classes in Python

Michele Simionato mis6 at pitt.edu
Fri Apr 18 17:08:11 EDT 2003 
Bill Martin <wcmartin at vnet.net> wrote in message news:<3EA01A9B.4030209 at vnet.net>...
> I'm wondering about the value of allowing a class definition like this:
> 
>              class C:
>                pass
> 
> Now I can define a = c() and b = c(), then say a.x1 = 1.2, b.x2 = 3.5 or 
> some such. If I try to print a.x2 or b.x1, I get an exception message 
> basically saying those member variables don't exist. It seems to me this 
> defeats one of the basic ideas behind OOP, that is to assign members to 
> the class definition.

Some time ago I posted this recipe to make Python anal in the way you
want:

  def frozen(self,name,value):
      if hasattr(self,name):
          object.__setattr__(self,name,value) # standard __setattr__
      else:
          raise AttributeError("You cannot add attributes to %s" % self)

  class Frozen(object):
      """Subclasses of Frozen are frozen, i.e. it is impossibile to add
       new attributes to them and their instances"""
      __setattr__ = frozen
      class __metaclass__(type):
          __setattr__ = frozen


 Cheers,


                  Michele
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