这是一个简单的解析器，使位置参数为可选(nargs =?).

Here's a simple parser that makes the positional argument optional (nargs=?).

In [171]: parser=argparse.ArgumentParser()
In [172]: parser.add_argument('func', nargs='?',default='foo')
In [173]: parser.add_argument('--list',action='store_true')
In [174]: args=parser.parse_args(['--list'])
In [175]: args
Out[175]: Namespace(func='foo', list=True)
In [176]: args=parser.parse_args(['bar'])
In [177]: args
Out[177]: Namespace(func='bar', list=False)
In [178]: parser.parse_args(['--list','bar'])
Out[178]: Namespace(func='bar', list=True)
In [179]: parser.parse_args([])
Out[179]: Namespace(func='foo', list=False)

不要为位置参数提供 dest 参数；它已经在第一个字符串中获得了一个. store_true 操作不需要默认设置；它会自动获取 False .

Don't give a positional argument a dest parameter; it already gets one in the first string. store_true action doesn't need a default; it gets False automatically.

通过此设置，您可以检查 args.list ；它为 True ，然后忽略 args.func 值.如果 False ，请使用 args.func .如果为 func 提供了有意义的默认值，则用户是否为您提供值都无关紧要.换句话说，它实际上不是必需的.(如果需要的话，定义默认值是没有意义的).或者，如果他们俩都给您，您可能会反对:

With this set up, you could check args.list; it True, then just ignore the args.func value. If False, use the args.func. If you have given func a meaningful default, if shouldn't matter whether the user gives you a value or not. In other words, it really isn't required. (And if required there's no point in defining a default). Or you could object if they give you both:

In [184]: args=parser.parse_args(['--list','bar'])
In [185]: if args.list and args.func!='foo':
       parser.error('you stupid user ...')
   .....:     
usage: ipython3 [-h] [--list] [func]
ipython3: error: you stupid user ...

我们可以进一步将这两个参数放在 mutually_exclusive_group 中，并使该组成为 required .这样的组接受这样的一个可选位置.我认为文档足够清晰，但是如果需要，我可以扩大答案.

We could go the further step of placing these 2 arguments in a mutually_exclusive_group, and making that group required. Such a group accepts one optional positional like this. I think the docs are clear enough, but I could expand my answer if needed.