code: def _create_payload(self, symbols):
          
          payload = {'access_key': self.access_key}
          if symbols is not None:
              payload['symbols'] = ','.join(symbols)
  
          return payload
docs: Creates a payload with no none values.
  
          :param symbols: currency symbols to request specific exchange rates.
          :type symbols: list or tuple
          :return: a payload.
          :rtype: dict