_Prevent_ dynamic attribute addition?

[Robert Oschler]

Python newbie here, using Python 2.2.2 on a SuSE Linux box.  There's a
feature in Python I find very powerful but a bit disconcerting, the ability
to add new attributes to a class object dynamically.  Is there a way to make
a class 'non-modifiable' in that sense, without losing the ability to
further derive from it?  Here's a scenario that describes my concern:

Let's say I have two classes A and B.  A has a data member called
NumberOfMonkeys and B does not.  I write tons of code and lots of it uses
A.NumberOfMonkeys.  Later I realize that NumberOfMonkeys really belongs in
Class B so I delete it from A's class declaration and move it into B's.  All
the dependent code that used to manipulate the value of the existing
NumberOfMonkeys attribute will now be adding the NumberOfMonkeys attribute
dynamically to Class A.  I would an error to be thrown instead so, so I can
fix/update the code.  I know I can check for the existence of an attribute
on an object, but that sounds pretty cumbersome to do that before any and
every use of an attribute.

Is there a way to make a class 'non-modifiable', or am I just not
understanding something here?

thx