:param param: parameter name to look for in request body
        :type param: str
        :rtype: str

def __get_param(self, param):
        """
        :param param: parameter name to look for in request body
        :type param: str
        :rtype: str
        """
        try:
            return self.request_json[param]
        except KeyError:
            raise FnexchangeError(400, 'Bad request: {0} is required'.format(param))