Setting options from environment variables when using argparse

You can set the default= of the argument to a .get() of os.environ with the environment variable you want to grab.

You can also pass a 2nd argument in the .get() call, which is the default value if .get() doesn't find an environment variable by that name (by default .get() returns None in that case).

import argparse
import os

parser = argparse.ArgumentParser(description='test')
parser.add_argument('--url', default=os.environ.get('URL'))

args = parser.parse_args()
if not args.url:
    exit(parser.print_usage())

I use this pattern frequently enough that I have packaged a simple action class to handle it:

import argparse
import os

class EnvDefault(argparse.Action):
    def __init__(self, envvar, required=True, default=None, **kwargs):
        if not default and envvar:
            if envvar in os.environ:
                default = os.environ[envvar]
        if required and default:
            required = False
        super(EnvDefault, self).__init__(default=default, required=required, 
                                         **kwargs)

    def __call__(self, parser, namespace, values, option_string=None):
        setattr(namespace, self.dest, values)

I can then call this from my code with:

import argparse
from envdefault import EnvDefault

parser=argparse.ArgumentParser()
parser.add_argument(
    "-u", "--url", action=EnvDefault, envvar='URL', 
    help="Specify the URL to process (can also be specified using URL environment variable)")
args=parser.parse_args()

I usually have to do this for multiple arguments (authentication and API keys).. this is simple and straight forward. Uses **kwargs.

def environ_or_required(key):
    return (
        {'default': os.environ.get(key)} if os.environ.get(key)
        else {'required': True}
    )

parser.add_argument('--thing', **environ_or_required('THING'))