python - Correct way to get allowed arguments from ArgumentParser

Question

Question: What is the intended / official way of accessing possible arguments from an existing argparse.ArgumentParser object?

Example: Let's assume the following context:

import argparse
parser = argparse.ArgumentParser()
parser.add_argument('--foo', '-f', type=str)

Here I'd like to get the following list of allowed arguments:

['-h', '--foo', '--help', '-f']

I found the following workaround which does the trick for me

parser._option_string_actions.keys()

But I'm not happy with it, as it involves accessing a _-member that is not officially documented. Whats the correct alternative for this task?

Answer 1

I don't think there is a "better" way to achieve what you want.

---

If you really don't want to use the _option_string_actions attribute, you could process the parser.format_usage() to retrieve the options, but doing this, you will get only the short options names.

If you want both short and long options names, you could process the parser.format_help() instead.

This process can be done with a very simple regular expression: -+w+

import re

OPTION_RE = re.compile(r"-+w+")
PARSER_HELP = """usage: test_args_2.py [-h] [--foo FOO] [--bar BAR]

optional arguments:
  -h, --help         show this help message and exit
  --foo FOO, -f FOO  a random options
  --bar BAR, -b BAR  a more random option
"""

options = set(OPTION_RE.findall(PARSER_HELP))

print(options)
# set(['-f', '-b', '--bar', '-h', '--help', '--foo'])

---

Or you could first make a dictionnary which contains the argument parser configuration and then build the argmuent parser from it. Such a dictionnary could have the option names as key and the option configuration as value. Doing this, you can access the options list via the dictionnary keys flattened with itertools.chain:

import argparse
import itertools

parser_config = {
    ('--foo', '-f'): {"help": "a random options", "type": str},
    ('--bar', '-b'): {"help": "a more random option", "type": int, "default": 0}
}

parser = argparse.ArgumentParser()
for option, config in parser_config.items():
    parser.add_argument(*option, **config)

print(parser.format_help())
# usage: test_args_2.py [-h] [--foo FOO] [--bar BAR]
# 
# optional arguments:
#   -h, --help         show this help message and exit
#   --foo FOO, -f FOO  a random options
#   --bar BAR, -b BAR  a more random option

print(list(itertools.chain(*parser_config.keys())))
# ['--foo', '-f', '--bar', '-b']

This last way is what I would do, if I was reluctant to use _option_string_actions.