def policy(self, resp_or_url, request):
        """
        Determine Referrer-Policy to use from a parent Response (or URL),
        and a Request to be sent.

        - if a valid policy is set in Request meta, it is used.
        - if the policy is set in meta but is wrong (e.g. a typo error),
          the policy from settings is used
        - if the policy is not set in Request meta,
          but there is a Referrer-policy header in the parent response,
          it is used if valid
        - otherwise, the policy from settings is used.
        """
        policy_name = request.meta.get('referrer_policy')
        if policy_name is None:
            if isinstance(resp_or_url, Response):
                policy_header = resp_or_url.headers.get('Referrer-Policy')
                if policy_header is not None:
                    policy_name = to_native_str(policy_header.decode('latin1'))
        if policy_name is None:
            return self.default_policy()

        cls = _load_policy_class(policy_name, warning_only=True)
        return cls() if cls else self.default_policy()

        Determine Referrer-Policy to use from a parent Response (or URL),
        and a Request to be sent.

        - if a valid policy is set in Request meta, it is used.
        - if the policy is set in meta but is wrong (e.g. a typo error),
          the policy from settings is used
        - if the policy is not set in Request meta,
          but there is a Referrer-policy header in the parent response,
          it is used if valid
        - otherwise, the policy from settings is used.
        
stackoverflow/venv/lib/python3.6/site-packages/scrapy/spidermiddlewares/referer.py