Python Argparse: Arguments Within Optional Arguments

Question

I am writing a script that can do multiple things, depending on the arguments passed into the command line. For example:

#changed and simplified from actual code
parser.add_argument('-a','--apple')
parser.add_argument('-b','--boy')
parser.add_argument('-c', '--car')
parser.add_argument('-d', '--dog')
args = parser.parse_args()

I only want 1 argument in the commandline. So, only one of -a,-b,-c,-d would be accepted, and then the code will execute based on whichever argument was passed.

I can do this by a series of if statements, like

if args.a:
if args.b:

etc. Is there a better way to approach this?

For each command line option (-a,-b,-c,-d), I would like to have some arguments passed in. These arguments would be specific and would vary depending on the initial command line option. For example:

python test.py -a apple aardvark aquaman

I considered using nargs, but I was not sure how to make each argument was

in the correct order and
the argument that is needed. In the above example, assuming the number of extra optional arguments would be 3, I want to make sure that all the arguments start with the letter a.

How could I approach this?

Bakuriu · Accepted Answer · 2019-02-18 22:35:26Z

1

You can define mutually exclusive options using add_mutually_exclusive_group():

group = parser.add_mutually_exclusive_group()
group.add_argument('-a','--apple')
group.add_argument('-b','--boy')
group.add_argument('-c', '--car')
group.add_argument('-d', '--dog')
args = parser.parse_args()

Regarding the second part you can specify nargs and a custom type. The type can simply be a function that takes the string of the argument:

def check_a(val):
    if not val or not val[0] == 'a':
        raise argparse.ArgumentTypeError('Must be a value starting with a')
    return val

parser.add_argument('-a', '--apple', nargs=3, type=check_a)

Regarding the order of the arguments passed to option apple AFAIK you have to check it afterswards:

args = parser.parse_args()
if is_wrong_order(args.apple):
    parser.error('Incorrect argument order to apple')

Anyway in your case you probably want to use subparsers instead of options.

answered Feb 18, 2019 at 22:35

Bakuriu

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3 Comments

helloworld95 Over a year ago

Thanks! I will try this out. Quick question, when you do "type=check_a", why are you not passing in a parameter when the function requires the argument "val"?

helloworld95 Over a year ago

Also, another question. I'm pretty new to everything about argparse. How would you access the return value of check_a? Would it be something like args.apple.type()?

Bakuriu Over a year ago

@helloworld95 I'm passing the function as an argument, I'm not calling it. argparse will call the function and store the return value as a list in args.apple as you can see in my last example.

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