@ndb.transactional<EOL>def mark_done(task_id):
task = Task.get_by_id(task_id)<EOL>if task is None:<EOL><INDENT>raise ValueError('<STR_LIT>' % task_id)<EOL><DEDENT>task.done = True<EOL>task.put()<EOL>
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.