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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def mark_done(task_id):
 """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. """

  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()