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.

• approxiblue: 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.
• goofd: 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

Answer 1

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

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

Answer 2

import sys
def main(args):
 print args[0]
 print len(args)
 print [arg for arg in args]
if __name__ == '__main__':
 main(sys.argv)

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