How to use the fastly.errors.AuthenticationError function in fastly

def purge_key(self, service, key, soft=False):
        if not isinstance(self.conn.authenticator, KeyAuthenticator):
            raise AuthenticationError("This request requires an API key")

        headers = {}
        if soft:
            headers['Fastly-Soft-Purge'] = 1

        resp, data = self.conn.request('POST', '/service/%s/purge/%s' % (service, key), headers=headers)
        return resp.status == 200