def sleep(self, response=None):
        """ Sleep between retry attempts.

        This method will respect a server's ``Retry-After`` response header
        and sleep the duration of the time requested. If that is not present, it
        will use an exponential backoff. By default, the backoff factor is 0 and
        this method will return immediately.
        """

        if response:
            slept = self.sleep_for_retry(response)
            if slept:
                return

        self._sleep_backoff()
 Sleep between retry attempts.

        This method will respect a server's ``Retry-After`` response header
        and sleep the duration of the time requested. If that is not present, it
        will use an exponential backoff. By default, the backoff factor is 0 and
        this method will return immediately.
        
stackoverflow/venv/lib/python3.6/site-packages/pip-19.0.3-py3.6.egg/pip/_vendor/urllib3/util/retry.py