Issue parsing multiple positional arguments

Question

I must be missing something really obvious, but I can't for the life of me spot it.

import argparse


def parse_args():
    parser = argparse.ArgumentParser(description="Copies selected files.")
    parser.add_argument(
        "-c", "--checksum", type=bool, default=False
    )
    parser.add_argument("source")
    parser.add_argument("target")
    return parser.parse_args()


def main():
    args = parse_args()
    print(args.checksum, args.source, args.target)


main()

>python file_proc.py source1 target1
False source1 target1

So far, so good.

>python file_proc.py -c source1 target1
usage: file_proc.py [-h] [-c CHECKSUM] source target
file_proc.py: error: the following arguments are required: target

Now, why doesn't it output True source1 target1? It must be right there in front of me, but I need another pair of eyes.

Thanks!

The type parameter is supposed to be a function, something that will transform the input string into a desired value. You have to write your own if you want to interpret strings like "yes" or "no" as True/False booleans. — hpaulj
– hpaulj, Commented Nov 5, 2020 at 16:39

Mayank Porwal · Accepted Answer · 2020-11-05 10:44:16Z

1

The way you've coded basically means, that if you are passing -c, then you need to pass a bool in the args as well.

Hence your statement:

>python file_proc.py source1 target1
False source1 target1

works fine, because you didn't pass -c. So, it printed False.

But when you pass -c in the command-line, it needs another variable to be passed in argument. So do this:

>python file_proc.py -c d source1 target1
True source1 target1

This means a bool was passed as an argument, hence it prints True.

edited Nov 5, 2020 at 10:44

answered Nov 5, 2020 at 10:39

Mayank Porwal

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Comments

MisterMiyagi · Accepted Answer · 2020-11-05 10:58:19Z

0

The argument declaration for --checksum means that following -c/--checksum, an actual boolean is passed in. For example, --checksum yes would "enable" the checksum; due to how bool interprets strings, any argument other than the empty string evaluates to True.

Use the store_true/store_false action to define an argument-less flag that is toggled on/off by being mentioned.

    parser.add_argument(
        "-c", "--checksum", action='store_true'
    )

answered Nov 5, 2020 at 10:58

MisterMiyagi

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1 Comment

mrgou Over a year ago

I know it was something basic! It's meant to be a switch argument, so I indeed needed action='store_true'. Thanks!

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