[FIXED] How to include Nargs from argparse in my code without getting errors?

Issue

I wrote cmd line interface to perform calculation on a number. If I enter using powershell the command:

python .\regex1.py *x 5 *y 6 *o mul

it prints:

In calc args
Something went wrong

However if i comment out Nargs it gives the expected results.

 entered thiscommand: python .\regex1.py *x 5 *y 6 *o mul
Got :mul
     30.0

Why is it so, why is Nargs creating this problem and how to solve it while keeping Nargs in our code? The code is below:

import argparse
import sys

def calc(args):
    print("In calc args")
    if str(args.o) == 'add':
        return args.x + args.y

    elif str(args.o) == 'mul':
        return args.x * args.y

    elif str(args.o) == 'sub':
        return args.x - args.y

    elif str(args.o) == 'div':
        return args.x / args.y

    else:
        return "Something went wrong"

if __name__ == '__main__':
    parser = argparse.ArgumentParser(prog= "utility Calculator",
                                     usage= "To perform calculation on an integer",
                                     description="Thsi is arguments help",
                                     epilog="Hope your problem is resolved, if not contact me",
                                     prefix_chars="*",
                                     argument_default= 1.0,
                                     add_help= True,
                                     allow_abbrev= True
                                     )
    parser.add_argument('*x',
                        type=float,
                        action="store",
                        nargs=1,
                        help="Enter first number. This is a utility for calculation. Please contact me",
                        metavar= "  First Number"
                        )

    parser.add_argument('*y',
                        type=float,
                        action="store",
                        nargs=1,
                        help="Enter Second number. This is a utility for calculation. Please contact me",
                        metavar= "  Second Number"
                        )

    parser.add_argument('*o',
                        type=str,
                        # action="store"
                        nargs=1,
                        default= "add",
                        help="Enter the operand."
                             "*mul for multiplication"
                             "*add for addition"
                             "*div for division"
                             "*sub for subtraction",
                        metavar= "  Operand"
                        )

    args = parser.parse_args()
    print(str(args.o), args.x, args.y)
    sys.stdout.write(str(calc(args)))

Solution

nargs serves a different purpose and I'm not sure why you are trying to use it here since your arguments contain only one value.

When nargs=1 your argument is actually a list of that single argument. If you remove the nargs=1 you get the argument itself. And looking at your code, all if statements fail and that's why you get Something went wrong.

Referencing the documentation where you can learn more about nargs:

N (an integer). N arguments from the command line will be gathered together into a list. For example:

>>> parser = argparse.ArgumentParser()
>>> parser.add_argument('--foo', nargs=2)
>>> parser.add_argument('bar', nargs=1)
>>> parser.parse_args('c --foo a b'.split())
Namespace(bar=['c'], foo=['a', 'b'])

Note that nargs=1 produces a list of one item. This is different from the default, in which the item is produced by itself.

Answered By - Vlad Siv