def total_output_token(self) -> Optional[int]:
        """
        Get the total number of output tokens used by this task.
        
        Returns:
            Optional[int]: The number of output tokens, or None if not available
        """
        the_total_cost = self.get_total_cost()
        if the_total_cost and "output_tokens" in the_total_cost:
            return the_total_cost["output_tokens"]
        return None

        Get the total number of output tokens used by this task.
        
        Returns:
            Optional[int]: The number of output tokens, or None if not available
        
src/upsonic/client/tasks/tasks.py