Freezing data members of a class!

[Sunil Hadap]

Hello, I hope this is not too trivial question

I want to stop the user from adding any more data members to instance of
a class than those added by the constructor.

Secondly I want to prevent the user from chaning type of the data member
(trough assignment) which gets defined only construction time. I have
done the following. Is this the best way!

class SuData:
    def __setattr__(self,attrname,value):
        print "__setattr__ for %s called with value " % attrname, value
        if self.__dict__.has_key(attrname):
            if type(self.__dict__[attrname]) is type(value):
                self.__dict__[attrname] = value
            else:
                raise AssertionError, attrname
        else:
            raise AssertionError, attrname
    def __delattr__(self,attrname):
        raise AssertionError, attrname

class SuNumericData(SuData):
    def __init__(self, initial):
        self.__dict__['_data'] = initial

Python is fun to dig into! Thanks a lot

Sunil