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

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

This answer suggests optparse which is appropriate for older Python versions. For Python 2.7 and above, argparse replaces optparse. See this answer for more information.

As other people pointed out, you are better off going with optparse over getopt. getopt is pretty much a one-to-one mapping of the standard getopt(3) C library functions, and not very easy to use.

optparse, while being a bit more verbose, is much better structured and simpler to extend later on.

Here's a typical line to add an option to your parser:

parser.add_option('-q', '--query',
            action="store", dest="query",
            help="query string", default="spam")

It pretty much speaks for itself; at processing time, it will accept -q or --query as options, store the argument in an attribute called query and has a default value if you don't specify it. It is also self-documenting in that you declare the help argument (which will be used when run with -h/--help) right there with the option.

Usually you parse your arguments with:

options, args = parser.parse_args()

This will, by default, parse the standard arguments passed to the script (sys.argv[1:])

options.query will then be set to the value you passed to the script.

You create a parser simply by doing

parser = optparse.OptionParser()

These are all the basics you need. Here's a complete Python script that shows this:

import optparse

parser = optparse.OptionParser()

parser.add_option('-q', '--query',
    action="store", dest="query",
    help="query string", default="spam")

options, args = parser.parse_args()

print 'Query string:', options.query

5 lines of python that show you the basics.

Save it in sample.py, and run it once with

python sample.py

and once with

python sample.py --query myquery

Beyond that, you will find that optparse is very easy to extend. In one of my projects, I created a Command class which allows you to nest subcommands in a command tree easily. It uses optparse heavily to chain commands together. It's not something I can easily explain in a few lines, but feel free to browse around in my repository for the main class, as well as a class that uses it and the option parser

Using docopt

Since 2012 there is a very easy, powerful and really cool module for argument parsing called docopt. Here is an example taken from its documentation:

"""Naval Fate.

Usage:
  naval_fate.py ship new <name>...
  naval_fate.py ship <name> move <x> <y> [--speed=<kn>]
  naval_fate.py ship shoot <x> <y>
  naval_fate.py mine (set|remove) <x> <y> [--moored | --drifting]
  naval_fate.py (-h | --help)
  naval_fate.py --version

Options:
  -h --help     Show this screen.
  --version     Show version.
  --speed=<kn>  Speed in knots [default: 10].
  --moored      Moored (anchored) mine.
  --drifting    Drifting mine.

"""
from docopt import docopt


if __name__ == '__main__':
    arguments = docopt(__doc__, version='Naval Fate 2.0')
    print(arguments)

So this is it: 2 lines of code plus your doc string which is essential and you get your arguments parsed and available in your arguments object.

Using python-fire

Since 2017 there's another cool module called python-fire. It can generate a CLI interface for your code with you doing zero argument parsing. Here's a simple example from the documentation (this small program exposes the function double to the command line):

import fire

class Calculator(object):

  def double(self, number):
    return 2 * number

if __name__ == '__main__':
  fire.Fire(Calculator)

From the command line, you can run:

> calculator.py double 10
20
> calculator.py double --number=15
30