code: def _add_referrer(cls, request: Request, url_record: URLRecord):
          '''Add referrer URL to request.'''
          # Prohibit leak of referrer from HTTPS to HTTP
          # rfc7231 section 5.5.2.
          if url_record.parent_url.startswith('https://') and \
                  url_record.url_info.scheme == 'http':
              return
  
          request.fields['Referer'] = url_record.parent_urf _add_referrer(cls, request: Request, url_record: URLRecord):
          '''Add referrer URL to request.'''
          # Prohibit leak of referrer from HTTPS to HTTP
          # rfc7231 section 5.5.2.
          if url_record.parent_url.startswith('https://') and \
                  url_record.url_info.scheme == 'http':
              return
  
          request.fields['Referer'] = url_record.parent_url
docs: Add referrer URL to request.