Private attribute

[Ken Starks]

I have a class with an attribute called 'gridsize' and I want
a derived class to force and keep it at 0.8 (representing 8mm).

Is this a correct, or the most pythonic approach?

####################

     def __getattr__(self,attrname):
         if attrname == 'gridsize':
             return 0.8

     def __setattr__(self,attrname,value):
         if attrname == 'gridsize':
             pass
         else:
             self.__dict__[attrname]=value

#####################


Cheers,
Ken.