argparse: Ignore positional arguments if a flag is set?

user_5646852 0
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I want to provide the command with 3 arguments: <version>, <input file> and <output file> under normal usage. Except there's a specific flag --init, which will basically run the program without any input and output file specifications required.

I would therefore ideally have a command which usage is:

py program.py --init

OR

py program.py <version> <input file> <output file>

However, since positional arguments are always required (because all 3 are required in any other circumstance than --init), there seems to be no way to cleanly get this syntax and all I could think of would be to make the 3 positional arguments into an optional flag, raise an exception if the optional flag isn't there when --init is not called. And that all seems ugly.

My code so far:

def get_args():
    parser = argparse.ArgumentParser(description="Tool for FOO-ing a BAR.")
    parser.add_argument(dest="version", help="The version.")
    parser.add_argument(dest="input", help="The input file.")
    parser.add_argument(dest="output", help="The output file.")

    parser.add_argument("-i", "--init", dest="init", action="store_true", help="Foo Init.")

    return parser.parse_args()

To Clarify:
Either all 3 arguments (<version> <input> <output>) MUST be specified.
OR
The program is only ran with --init flag and 0 arguments should be specified.

The program should NOT accept with 0, 1 or 2 arguments specified (without the --init flag).

argparse python
4 Answers

You can define your own action class:

class init_action(argparse.Action):
    def __init__(self, option_strings, dest, **kwargs):
        return super().__init__(option_strings, dest, nargs=0, default=argparse.SUPPRESS, **kwargs)
    
    def __call__(self, parser, namespace, values, option_string, **kwargs):
        # Do whatever should be done here
        parser.exit()

def get_args():
    parser = argparse.ArgumentParser(description="Tool for FOO-ing a BAR.")
    parser.add_argument(dest="version", help="The version.")
    parser.add_argument(dest="input", help="The input file.")
    parser.add_argument(dest="output", help="The output file.")

    parser.add_argument("-i", "--init", action=init_action, help="Foo Init.")

    return parser.parse_args()
user_93959220

I had the exact same problem, and fixed it by adding nargs="?" to all my positional arguments

def get_args():
    parser = argparse.ArgumentParser(description="Tool for FOO-ing a BAR.")
    parser.add_argument(dest="version", nargs="?", help="The version.")
    parser.add_argument(dest="input", nargs="?", help="The input file.")
    parser.add_argument(dest="output", nargs="?", help="The output file.")

    parser.add_argument("-i", "--init", dest="init", action="store_true", help="Foo Init.")

    return parser.parse_args()

but I'm not completely satisfied with it since when you run

py program.py -h

the synopsys is printed as

usage: program.py [-h] [--init] [version] [input] [output]

while I would want it to be

usage: program.py [-h] [--init] version input output

since version, input and output are required, just that they are not required when you use the --init flag.

Another downside is that you do not get the help text printed if you do not provide the positional arguments correctly.

I think it is sad that argparse have no way to set ignore_positional_args=True or something on flags. Especially since this property already is true for the -h flag. At least I'm not able to find a way to do it.

EDIT:

I actually found a way to better way to do it, but put it in a separate answer because I think this can be a useful approach as well.

user_93959220

parser.add_argument has a default parameter (docs) which can be used here for version, input and output parameters. Now you won't need third init param

user_92766340

One possible solution is to use sys.argv to determine whether the flag is enabled. Using your example,

import sys

if '--init' not in sys.argv:
    parser.add_argument(dest="version", help="The version.")
    parser.add_argument(dest="input", help="The input file.")
    parser.add_argument(dest="output", help="The output file.")

Argparse will allow you to run the program with either --init flag without any arguments or force you to include other arguments if --init flag is not present.

One downside to this approach is that you will see the following output, when you run py program.py -h:

usage: program.py [--init] version input output

positional arguments:
  version - The version.
  input - The input file.
  output - The output file.

optional arguments:
  --init - ...

which makes it hard for the user to understand the logic unless you clarify it in the description in the help section.

user_123268370
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