def get_retry_after(self, response):
        """ Get the value of Retry-After in seconds. """

        retry_after = response.getheader("Retry-After")

        if retry_after is None:
            return None

        return self.parse_retry_after(retry_after)
 Get the value of Retry-After in seconds. 
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_vendor/urllib3/util/retry.py