'Return the domain object associated with the domain name if it is
present in this dictionary. Otherwise, set the value for the
domain name to the default and return that value.
@type key: L{bytes}
@param key: A domain name.
@type default: L{IDomain} provider
@param default: A domain object.
@rtype: L{IDomain} provider or L{None}
@return: The domain object associated with the domain name.'

def setdefault(self, key, default):

    return self.domains.setdefault(key, default)
