argparse声明参数althoug指定的默认值

Question

我写了

parser.add_argument("n_tasks", default=20, help='Number of tasks to start')

并且期待，参数的缺席将默认为20。 Althoug写道

usage: get_sentinel_spawner.py [-h] n_tasks
get_sentinel_spawner.py: error: the following arguments are required: n_tasks

为什么？