Argparse

What is Argparse?

Argparse is a parser for command-line options, arguments and subcommands. This library makes it easy to write user-friendly command-line interfaces by:

defines what arguments it requires
and figuring out how to parse those out of sys.argv.

The argparse module automatically generates help and usage messages and issues errors when users give the program invalid arguments.

Getting Started

See agrparse_1.py

# import the library
import argparse

# initialize the parser
parser = argparse.ArgumentParser()

# parse arguments from sys.argv
parser.parse_args()

# run the code and ask for help
!python argparse_1.py --help

usage: argparse_1.py [-h]

optional arguments:
  -h, --help  show this help message and exit

Positional Arguments

See argparse_2.py

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("name")
parser.add_argument("age")
parser.add_argument("city")
args = parser.parse_args()

print(args.name, args.age, args.city)

!python argparse_2.py --help

usage: argparse_2.py [-h] name age city

positional arguments:
  name
  age
  city

optional arguments:
  -h, --help  show this help message and exit

!python argparse_2.py Ben 37 Pittsburgh

Ben 37 Pittsburgh

!python argparse_2.py Brian "??" Pittsburgh

Brian ?? Pittsburgh

Extending the help text

See argparse_3.py

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("name", help="the name of the person you want to find")
parser.add_argument("age", help="the age of the person you'd like to find")
parser.add_argument("city", help="the city you'd like to search")

args = parser.parse_args()

!python argparse_3.py --help

usage: argparse_3.py [-h] name age city

positional arguments:
  name        the name of the person you want to find
  age         the age of the person you'd like to find
  city        the city you'd like to search

optional arguments:
  -h, --help  show this help message and exit

Changing the default argument type

Argparse treats all arguments as strings by default. You can change the expected data type when you add each argument.

See argparse_4.py

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("name", help="the name of the person you want to find")
parser.add_argument("age", help="the age of the person you'd like to find", type=int)
parser.add_argument("city", help="the city you'd like to search")

args = parser.parse_args()

!python argparse_4.py Ben abc Pittsburgh

usage: argparse_4.py [-h] name age city
argparse_4.py: error: argument age: invalid int value: 'abc'

!python argparse_4.py Ben 37 Pittsburgh

Optional arguments

See argparse_5.py

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("--verbose", 
                    help="increase output verbosity",
                    action="store_true")

args = parser.parse_args()

if args.verbose:
    print("verbosity turned on")

An optional argument (or option) is (by default) given None as a value when its not being used.

Using the –verbosity option, only two values are actually useful, True or False.
The keyword “action” is being given the value “store_true” which means that if the option is specifed, then assign the value “True” to args.verbose
Not specifying the option implies False.

!python argparse_5.py --help

usage: argparse_5.py [-h] [--verbose]

optional arguments:
  -h, --help  show this help message and exit
  --verbose   increase output verbosity

!python argparse_5.py --verbose

verbosity turned on

!python argparse_5.py

Short options

See argparse_6.py

import argparse

parser = argparse.ArgumentParser()

parser.add_argument("-n", "--name", help="the name of the person you want to find")
parser.add_argument("-a", "--age", help="the age of the person you'd like to find", type=int)
parser.add_argument("-c", "--city", help="the city you'd like to search")
parser.add_argument("-v", "--verbose", help="increase output verbosity", action="store_true")

args = parser.parse_args()

if args.verbose:
    print(f"Searching for {args.name} {args.age} years of age in or around {args.city}")
else:
    print(f"Searching for {args.name}")

!python argparse_6.py --help

usage: argparse_6.py [-h] [-n NAME] [-a AGE] [-c CITY] [-v]

optional arguments:
  -h, --help            show this help message and exit
  -n NAME, --name NAME  the name of the person you want to find
  -a AGE, --age AGE     the age of the person you'd like to find
  -c CITY, --city CITY  the city you'd like to search
  -v, --verbose         increase output verbosity

!python argparse_6.py -n Ben -a 37 -c Pittsburgh

Searching for Ben

!python argparse_6.py --name Ben --age 37 --city Pittsburgh

Searching for Ben

!python argparse_6.py --name Ben --age 37 --city Pittsburgh --verbose

Searching for Ben 37 years of age in or around Pittsburgh