How to use argparse module on Python3

Learning using argparse module on Python3

Make an argparse.py file

At first, make a file that name is argparse.py file by any text editor.

Open the official document on your browser

Aliases
Japanese

Argparse チュートリアル — Python 3.6.5 ドキュメント

English

Argparse Tutorial — Python 3.6.5 documentation

Make the most simple script

Code is bellow.

# coding: utf-8
import argparse
parser = argparse.ArgumentParser()
parser.parse_args()

Show help.

$ python3 argparse.py -h
usage: argparse.py [-h]

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

Add positional arguments

Make positional arguments.
Code

# coding: utf-8
import argparse
parser = argparse.ArgumentParser()
# Positional arguments.
parser.add_argument("echo")
args = parser.parse_args()
print(args)

Show help message.

$ python3 argparse.py -h
usage: argparse.py [-h] echo

positional arguments:
  echo

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

$

Run the script with foo in positional arguments.

$ python3 argparse.py foo
Namespace(echo='foo')
$

Check what functions in ArgumentParser()

Check in interpreter mode.

$ ipython
Python 3.6.4 (default, Jan  6 2018, 11:51:15) 
Type 'copyright', 'credits' or 'license' for more information
IPython 6.2.1 -- An enhanced Interactive Python. Type '?' for help.

In [1]: import argparse

In [2]: parser = argparse.ArgumentParser()

In [3]: dir(parser)
Out[3]: 
['__class__',
 '__delattr__',
 '__dict__',
 '__dir__',
 '__doc__',
 '__eq__',
 '__format__',
 '__ge__',
 '__getattribute__',
 '__gt__',
 '__hash__',
 '__init__',
 '__init_subclass__',
 '__le__',
 '__lt__',
 '__module__',
 '__ne__',
 '__new__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__subclasshook__',
 '__weakref__',
 '_action_groups',
 '_actions',
 '_add_action',
 '_add_container_actions',
 '_check_conflict',
 '_check_value',
 '_defaults',
 '_get_args',
 '_get_formatter',
 '_get_handler',
 '_get_kwargs',
 '_get_nargs_pattern',
 '_get_option_tuples',
 '_get_optional_actions',
 '_get_optional_kwargs',
 '_get_positional_actions',
 '_get_positional_kwargs',
 '_get_value',
 '_get_values',
 '_handle_conflict_error',
 '_handle_conflict_resolve',
 '_has_negative_number_optionals',
 '_match_argument',
 '_match_arguments_partial',
 '_mutually_exclusive_groups',
 '_negative_number_matcher',
 '_option_string_actions',
 '_optionals',
 '_parse_known_args',
 '_parse_optional',
 '_pop_action_class',
 '_positionals',
 '_print_message',
 '_read_args_from_files',
 '_registries',
 '_registry_get',
 '_remove_action',
 '_subparsers',
 'add_argument',
 'add_argument_group',
 'add_help',
 'add_mutually_exclusive_group',
 'add_subparsers',
 'allow_abbrev',
 'argument_default',
 'conflict_handler',
 'convert_arg_line_to_args',
 'description',
 'epilog',
 'error',
 'exit',
 'format_help',
 'format_usage',
 'formatter_class',
 'fromfile_prefix_chars',
 'get_default',
 'parse_args',
 'parse_known_args',
 'prefix_chars',
 'print_help',
 'print_usage',
 'prog',
 'register',
 'set_defaults',
 'usage']

In [4]: 

We can see a lot of functions.
And there is add_argument function.
There is add_argument() method document.

Japanese

16.4. argparse — コマンドラインオプション、引数、サブコマンドのパーサー — Python 3.6.5 ドキュメント

Engelish

16.4. argparse — Parser for command-line options, arguments and sub-commands — Python 3.6.5 documentation

Add help detail message.

Code changes...

# coding: utf-8
import argparse
parser = argparse.ArgumentParser()
# Positional arguments.
parser.add_argument("echo", help="echo the string you use here")
args = parser.parse_args()
print(args)

Only add help= option.

Run this.

$ python3 argparse.py -h
usage: argparse.py [-h] echo

positional arguments:
  echo        echo the string you use here

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

Added echo argument message. OK, we can add posisional argument message!

Add argument type in positional argument.

Code is bellow.

# coding: utf-8
import argparse
parser = argparse.ArgumentParser()
# Positional arguments.
parser.add_argument("echo_num", help="echo the number you use here", type=int)
args = parser.parse_args()
print(args)

Run this help.

$ python3 argparse.py -h
usage: argparse.py [-h] echo_num

positional arguments:
  echo_num    echo the number you use here

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

OK, it seems good.

Run the script with 2 that is positional argument.

$ python3 argparse.py 2
Namespace(echo_num=2)

$

OK, next. I will give test string to this script.

$ python3 argparse.py test
argparse.py: error: argument echo_num: invalid int value: 'test'

shell returned 2

$

I got an error with helpfull message. It's good behavior.

Use optional arguments.

Make optional argument sample.

# coding: utf-8
import argparse
parser = argparse.ArgumentParser()
# Optional arguments
parser.add_argument("--verbose", help="increase output verbosity")
args = parser.parse_args()
if args.verbose:
    print("verbosity turned on")

parser.add_argument() is same.
It is EASY!

Run the script.

$ python3 argparse.py -h
usage: argparse.py [-h] [--verbose VERBOSE]

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

Run the script with optional argument.

$ python3 argparse.py --verbose 1
verbosity turned on

args.verbose becomes True, run the print function.
It seems good!

Customize!

Change to get True or False on --verbose option. Now, --verbose opiton is able to get integer values.
Code bellow.

# coding: utf-8
import argparse
parser = argparse.ArgumentParser()
# Optional arguments
parser.add_argument("--verbose", help="increase output verbosity", action="store_true")
args = parser.parse_args()
if args.verbose:
    print("verbosity turned on")

Run the script with -h.

$ python3 argparse.py -h
usage: argparse.py [-h] [--verbose]

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

Help text has changed.

Short options

Easy

# coding: utf-8
import argparse
parser = argparse.ArgumentParser()
# Optional arguments
parser.add_argument("-v", "--verbose", help="increase output verbosity", action="store_true")
args = parser.parse_args()
if args.verbose:
    print("verbosity turned on")

Only add "-v" option. It's so easy!

Run the script with -h option.

$ python3 argparse.py -h
usage: argparse.py [-h] [-v]

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

Help message has changed!

Thank you!