code: def _add_mac_token(self, uri, http_method='GET', body=None,
                         headers=None, token_placement=AUTH_HEADER, ext=None, **kwargs):
          
          if token_placement != AUTH_HEADER:
              raise ValueError("Invalid token placement.")
  
          headers = tokens.prepare_mac_header(self.access_token, uri,
                                              self.mac_key, http_method, headers=headers, body=body, ext=ext,
                                              hash_algorithm=self.mac_algorithm, **kwargs)
          return uri, headers, body
docs: Add a MAC token to the request authorization header.
  
          Warning: MAC token support is experimental as the spec is not yet stable.