Question

Here is an example code:

import argparse

parser=argparse.ArgumentParser()
parser.add_argument('-main_arg')
subparser=parser.add_subparser()
a=subparser.add_parser('run')
a.add_argument('required_sub_arg')
a.add_argument('arg_a')
b=subparser.add_parser('b')
parser.parse_args()

I want it to read in -main_arg if I enter program run required_sub_arg -main_arg -arg_a

Right now, it doesn’t recognize -main_arg as a valid argument.

Answer 1

PSA to recent readers

As this question still has visits in 2018, before doing anything this complex with argparse, please consider using docopt or click instead. It will improve both your sanity and that of anyone who might read or modify your code. Thank you.

Original answer

As is, you have a few issues.

First, parser.parse_args is a method that returns a namespace of parser‘s arguments, so you should do something like

args = parser.parse_args()

Then args.main_args to get-main_arg from a call like

program -main_arg run required_sub_arg -arg_a

---

Your issue with main_arg is that you have created a argument to parser named main_arg, and you make a call like

program run required_sub_arg -main_arg -arg_a

that refers to an argument to a named main_arg. Since a doesn’t have such an argument, it is invalid.

In order to refer to a parser’s argument from one of its subparser, you have to make said subparser inherit the arguments of its parent. This is done with

a=parser.add_subparser('run', parents=[parser])

You have mistaken subparser for child parser. See http://docs.python.org/dev/py3k/library/argparse.html and https://code.google.com/p/argparse/issues/detail?id=54 for more informations.