What's the best way to parse command line arguments?

Question

What's the easiest, tersest, and most flexible method or library for parsing Python command line arguments?

Answer 1

argparse is the way to go. Here is a short summary of how to use it:

1) Initialize

import argparse

# Instantiate the parser
parser = argparse.ArgumentParser(description='Optional app description')

2) Add Arguments

# Required positional argument
parser.add_argument('pos_arg', type=int,
                    help='A required integer positional argument')

# Optional positional argument
parser.add_argument('opt_pos_arg', type=int, nargs='?',
                    help='An optional integer positional argument')

# Optional argument
parser.add_argument('--opt_arg', type=int,
                    help='An optional integer argument')

# Switch
parser.add_argument('--switch', action='store_true',
                    help='A boolean switch')

3) Parse

args = parser.parse_args()

4) Access

print("Argument values:")
print(args.pos_arg)
print(args.opt_pos_arg)
print(args.opt_arg)
print(args.switch)

5) Check Values

if args.pos_arg > 10:
    parser.error("pos_arg cannot be larger than 10")

Usage

Correct use:

$ ./app 1 2 --opt_arg 3 --switch

Argument values:
1
2
3
True

Incorrect arguments:

$ ./app foo 2 --opt_arg 3 --switch
usage: convert [-h] [--opt_arg OPT_ARG] [--switch] pos_arg [opt_pos_arg]
app: error: argument pos_arg: invalid int value: 'foo'

$ ./app 11 2 --opt_arg 3
Argument values:
11
2
3
False
usage: app [-h] [--opt_arg OPT_ARG] [--switch] pos_arg [opt_pos_arg]
convert: error: pos_arg cannot be larger than 10

Full help:

$ ./app -h

usage: app [-h] [--opt_arg OPT_ARG] [--switch] pos_arg [opt_pos_arg]

Optional app description

positional arguments:
  pos_arg            A required integer positional argument
  opt_pos_arg        An optional integer positional argument

optional arguments:
  -h, --help         show this help message and exit
  --opt_arg OPT_ARG  An optional integer argument
  --switch           A boolean switch