How to use the argument parser - argparse (Python)?

Nov 24, 2017 |

Argparse is a module that allows for neat parsing of options and arguments for python programs.

1. Positional arguments

A positional argument is an argument that is not supplied as a key=value pair. The example below shows how to use the argument parser with positional arguments.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
#################
#substract.py
#################
import argparse

def substract(a, b):
    print(a - b)

if __name__ == "__main__":
    parser = argparse.ArgumentParser(description="this program substracts 2 elements")
    #Create arguments: add_argument(name_of_argument, help_text, type_of_argument)
    parser.add_argument("a", help="1st element", type=int)
    parser.add_argument("b", help="2nd element", type=int)
    #Get arguments from the parser
    args =  parser.parse_args()
    substract(args.a, args.b)

We now run the file from the terminal:

1
2
$ python substract.py 2 3
$ -1

Note that the order of the argument matters:

python substract.py 2 3 returns -1
python substract.py 3 2 returns 1

If a Positional argument is not specified, it is automatically given the value None.

2. Optional arguments

Optional arguments are, as the name suggests, optional: they are not needed to run the program. For example, -h helper is a (built-in) optional argument.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
#################
#substract.py
#################
import argparse

def substract(a, b):
    print(a - b)

if __name__ == "__main__":
    parser = argparse.ArgumentParser(description="this program substracts 2 elements")
    # add_argument("short name", "long name", action="action to be done when the option is called ")
    # action="store_true" ==> set -v = True
    parser.add_argument("-v", "--verbose", help="print out", action="store_true")
    parser.add_argument("a", help="help text", type=int)
    parser.add_argument("b", help="help text", type=int)
    #Get arguments from the parser
    args =  parser.parse_args()

    if args.verbose:
        print("Substraction: {}-{}".format(args.a, args.b))
    add(args.a, args.b)

We then run the file from the terminal:

1
2
$ python substract.py 2 3 -v
$ -1

We can write the flag in any order we want. The argument -v can be called at any position. The following calls are equivalent:

python substract.py 2 3 -v
python substract.py 2 -v 3
python substract.py -v 2 3

store_true: the default is False but when the flag is called, it is given the value True.

An optional argument can be requested to be called by using the attribute required=True:

parser.add_argument("-v", "--verbose", help="help text", action="store_true", required=True)

3. Mutually exclusive argument

Mutually exclusive arguments are arguments that cannot be called more than one at a time. In the example below, we will add 2 mutually exclusive arguments: quiet and verbose.

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
#################
#substract.py
#################
import argparse

def substract(a, b):
    return a - b

if __name__ == "__main__":
    parser = argparse.ArgumentParser(description="describe what the program does")
    parser.add_argument("a", help="help text", type=int)
    parser.add_argument("b", help="help text", type=int)
    #create a container
    group = parser.add_mutually_exclusive_group()
    # '-q': short hand notation, '--quiet': long hand notation
    group.add_argument("-q", "--quiet", action="store_true", help="print quiet")
    group.add_argument("-v", "--verbose", action="store_true", help="print verbose")
    args =  parser.parse_args()

    if args.verbose:
        print("Substraction: {}-{} = {}".format(args.a, args.b, substract(args.a, args.b)))
    elif args.quiet:
        print("{}".format( substract(args.a, args.b) ))
    else:
        print("{}-{}={}".format(args.a, args.b, substract(args.a, args.b)))

Let’s run the code:

1
2
3
4
5
6
7
8
$ python substract.py 2 3
$ 2 - 3 = -1

$ python substract.py 2 3 -v
$ Substraction: 2 - 3 = -1

$ python substract.py 2 3 -q
$ -1