Python argparse select a list from choices

Question

How do you use argparse to provide a list of arguments from a group of choices. For example, lets say I want something like:

python sample.py --p1 ['a','b'] --p2 ['x','y']

Where p1 can only be any or all from list of 'a', 'b', 'c' and p2 can only be any or all from list of 'x', 'y', 'z'

Answer 1

I found a way to get the behavior you want, but with a different syntax than what you present. You have to specify each choice with a unique parameter name/value pair. If that's ok, then the following works:

parser = argparse.ArgumentParser(prog='game.py')
parser.add_argument('--p1', choices=['a', 'b', 'c'], action='append')
args = parser.parse_args(['--p1', 'a', '--p1', 'b'])
print(args)

Result:

Namespace(p1=['a', 'b'])

but this fails appropriately:

parser = argparse.ArgumentParser(prog='game.py')
parser.add_argument('--p1', choices=['a', 'b', 'c'], action='append')
args = parser.parse_args(['--p1', 'a', '--p1', 'b', '--p1', 'x'])
print(args)

Result:

usage: game.py [-h] [--p1 {a,b,c}]
game.py: error: argument --p1: invalid choice: 'x' (choose from 'a', 'b', 'c')

I can find nothing in the docs that suggests that ArgumentParser will take a list of valid values as a parameter value, which is what your version would require.

Answer 2

Using nargs with choices:

In [1]: import argparse
In [2]: p = argparse.ArgumentParser()
In [3]: p.add_argument('-a',nargs='+', choices=['x','y','z'])
Out[3]: _StoreAction(option_strings=['-a'], dest='a', nargs='+', const=None, default=None, type=None, choices=['x', 'y', 'z'], help=None, metavar=None)

In [4]: p.parse_args('-a x y z x x'.split())
Out[4]: Namespace(a=['x', 'y', 'z', 'x', 'x'])

In [5]: p.parse_args('-a x y z w x'.split())
usage: ipython3 [-h] [-a {x,y,z} [{x,y,z} ...]]
ipython3: error: argument -a: invalid choice: 'w' (choose from 'x', 'y', 'z')