argparse：如何配置多项选择，多值，可选参数？

Question

I'm trying to set up an argument that accepts one or more values from a given list of choices, but is not obligatory. I'm trying this (with a couple of variants that also don't work as expected):

parser.add_argument("FLAGS", nargs='*', choices=["X","Y","Z","ALL"])

I expect to get a list of values from the list of choices, or an empty list if nothing was given (that, I think, should be enforced by nargs='*' ). But regardless of whether I add default="" or not, when I don't pass any argument it fails with:

error: argument FLAGS: invalid choice: []

How to achieve what I need?

Answer 1

This probably doesn't fit your needs, but you can do it easily with an option like --flags .

parser.add_argument(
    "--flags",
    nargs='*',
    default=[],  # Instead of "None"
    choices=["X", "Y", "Z", "ALL"])

args = parser.parse_args()
print(args)

$ tmp.py
Namespace(flags=[])

$ tmp.py --flags
Namespace(flags=[])

$ tmp.py --flags X
Namespace(flags=['X'])

$ tmp.py --flags X Z
Namespace(flags=['X', 'Z'])

$ tmp.py --flags foobar
usage: tmp.py [-h] [--flags [{X,Y,Z,ALL} ...]]
tmp.py: error: argument --flags: invalid choice: 'foobar' (choose from 'X', 'Y', 'Z', 'ALL')

$ tmp.py --help
usage: tmp.py [-h] [--flags [{X,Y,Z,ALL} ...]]

optional arguments:
  -h, --help            show this help message and exit
  --flags [{X,Y,Z,ALL} ...]

Answer 2

A positional '*' gets some special handling. Its nargs is satisfied with an empty list (nothing). It's always handled. What does it mean to check an empty list of strings against the choices ?

So the get_values() method does:

    # when nargs='*' on a positional, if there were no command-line
    # args, use the default if it is anything other than None
    elif (not arg_strings and action.nargs == ZERO_OR_MORE and
          not action.option_strings):
        if action.default is not None:
            value = action.default
        else:
            value = arg_strings
        self._check_value(action, value)

where _check_value tests if value is in the choices .

Such a positional is best used with a valid default .

In [729]: p=argparse.ArgumentParser()
In [730]: a=p.add_argument("FLAGS", nargs='*', choices=["X","Y","Z","ALL"])
In [731]: p.parse_args([])
usage: ipython3 [-h] [{X,Y,Z,ALL} [{X,Y,Z,ALL} ...]]
ipython3: error: argument FLAGS: invalid choice: [] (choose from 'X', 'Y', 'Z', 'ALL')
...

Testing an empty list against choices fails:

In [732]: a.choices
Out[732]: ['X', 'Y', 'Z', 'ALL']
In [733]: [] in a.choices
Out[733]: False
In [734]: 'X' in a.choices
Out[734]: True

If we set a valid default:

In [735]: a.default='X'
In [736]: p.parse_args([])
Out[736]: Namespace(FLAGS='X')

This behavior is part of what allows us to use such a positional in a mutually_exclusive_group .

If you don't want to specify a valid default, then changing this to a flagged argument avoids the problem.