code: def set_arg(self, value):
          
          named_arg = '{0}A{1}'.format(self.arg_prefix, self.arg_index)
          self.args[named_arg] = value
          self.arg_index += 1
          return named_arg
docs: Set the query param in self.args based on the prefix and arg index
          and auto increment the arg_index
  
          :return: the string placeholder for the arg
          :rtype: str