def construct_auth (self):
        """Construct HTTP Basic authentication credentials if there
        is user/password information available. Does not overwrite if
        credentials have already been constructed."""
        if self.auth:
            return
        _user, _password = self.get_user_password()
        if _user is not None and _password is not None:
            self.auth = (_user, _password)
Construct HTTP Basic authentication credentials if there
        is user/password information available. Does not overwrite if
        credentials have already been constructed.