Parsing command line arguments with argparse

Question

I would like my python program to parse command like arguments like this:

python dosomething.py z a1 b1 a2 b2 ...

where I can have any number of a# b# pairs and z is an unrelated number. If necessary I am OK with specifying the number of a and b pairs.

I'm using argparse.

I guess you could read z and get it out of the way, then read the rest of arguments in order (stackoverflow.com/questions/9027028/argparse-argument-order) and error out if their number is odd. — approxiblue
– approxiblue, Commented Jul 2, 2013 at 4:53
you should be able to read the rest using nargs='+' and then can use a custom action to check for even pairs. and as @deshko mentioned the first positional argument can be specified as z with nargs=1 check stackoverflow.com/questions/13174975/… for the choice on nargs — goofd
– goofd, Commented Jul 2, 2013 at 4:58

FMc · Accepted Answer · 2013-07-02 05:20:59Z

3

You'll need to define a custom action for such specialized behavior.

import sys
import argparse

class AbsAction(argparse.Action):
    def __call__(self, parser, namespace, values, option_string=None):
        if len(values) % 2 == 0:
            # If valid, store the values.
            setattr(namespace, self.dest, values)

            # You could convert the flat list to a list of 2-tuples, if needed:
            #   zip(values[::2], values[1::2])
        else:
            # Otherwise, invoke a parser error with a message.
            parser.error('abs must be supplied as pairs')


ap = argparse.ArgumentParser()
ap.add_argument('z')
ap.add_argument('abs', nargs = '+', action = AbsAction)
opt = ap.parse_args()
print opt

edited Jul 2, 2013 at 5:20

answered Jul 2, 2013 at 5:14

FMc

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1 Comment

deltap Over a year ago

Thanks, I combined that with a slight hack to parse the 'a' 'b' combinations and now errors are handled gracefully.

Zangetsu · Accepted Answer · 2013-07-02 04:53:06Z

0

import sys

def main(args):
    print args[0]
    print len(args)
    print [arg for arg in args]

if __name__ == '__main__':
    main(sys.argv)

answered Jul 2, 2013 at 4:53

Zangetsu

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1 Comment

deltap Over a year ago

Thank you for the suggestion, I would like to stick with argparse if possible.

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