Set default value with argparse and same dest attribute

Question

here is a simple example.

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('-a', action = 'append_const', dest = 'letter', const = 'a')
parser.add_argument('-b', action = 'append_const', dest = 'letter', const = 'b')
parser.set_defaults(letter = ['a', 'b'])
print(parser.parse_args('-a'.split()))
print(parser.parse_args('-b'.split()))
print(parser.parse_args(''.split()))

Results :

Namespace(letter=['a', 'b', 'a'])
Namespace(letter=['a', 'b', 'b'])
Namespace(letter=['a', 'b'])

Without the set_defaults line, results are :

Namespace(letter=['a'])
Namespace(letter=['b'])
Namespace(letter=None)

How is it possible to configure argparse for such results ?

Namespace(letter=['a'])
Namespace(letter=['b'])
Namespace(letter=['a', 'b'])

unutbu · Accepted Answer · 2013-01-01 13:16:17Z

1

As others have mentioned, store_const almost does what you want, except that (as @Theodros Zelleke points out) -ab would not be parsed correctly. I think the simplest way to address that problem is to simply handle the case of no arguments after parse_args has been called:

import argparse
import sys

def parse_args(argv = sys.argv[1:]):
    parser = argparse.ArgumentParser()
    parser.add_argument('-a', action = 'append_const', dest = 'letter', const = 'a')
    parser.add_argument('-b', action = 'append_const', dest = 'letter', const = 'b')
    args = parser.parse_args(argv)
    if args.letter is None:
        args.letter = ['a','b']
    return args

print(parse_args('-a'.split()))
print(parse_args('-b'.split()))
print(parse_args('-ab'.split()))
print(parse_args(''.split()))

yields

Namespace(letter=['a'])
Namespace(letter=['b'])
Namespace(letter=['a', 'b'])
Namespace(letter=['a', 'b'])

answered Jan 1, 2013 at 13:16

unutbu

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1 Comment

MB4E Over a year ago

This is the solution I used. So, there is no solution using argparse defaults possiblities.

Cubi73 · Accepted Answer · 2020-02-13 15:43:32Z

0

You should store the const instead of appending it, use the action 'store_const' (doc):

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('-a', action = 'store_const', dest = 'letter', const = 'a')
parser.add_argument('-b', action = 'store_const', dest = 'letter', const = 'b')
parser.set_defaults(letter = ['a', 'b'])
print(parser.parse_args('-a'.split()))
print(parser.parse_args('-b'.split()))
print(parser.parse_args(''.split()))

This will result in:

Namespace(letter='a')
Namespace(letter='b')
Namespace(letter=['a', 'b'])

edited Feb 13, 2020 at 15:43

Cubi73

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answered Jan 1, 2013 at 12:43

zenpoy

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