Python dict as a class

ExDict.py

class ExDict(dict):
    """
        Extended Dictionary
    """
    def __init__(self,*arg,**kw):
          super( ExDict , self).__init__(*arg, **kw)

    def __setattr__(self, k, v):
        if k in self.keys():
            self[k] = v
        elif not hasattr(self, k):
            self[k] = v
        else:
            raise AttributeError, "Cannot set '%s', cls attribute already exists" % ( k, )

    def __getattr__(self, k):
        if k in self.keys():
            return self[k]
        raise AttributeError, "Cannot fetch '%s', cls attribute does not exist" % ( k, )