code: def mark_done(task_id):
    
    task = Task.get_by_id(task_id)
    if task is None:
      raise ValueError('Task with id %d does not exist' % task_id)
    task.done = True
    task.put()
docs: Marks a task as done.
  
    Args:
      task_id: The integer id of the task to update.
  
    Raises:
      ValueError: if the requested task doesn't exist.