Using env vars in argparse

demo.py

import argparse

from EnvDefault import env_default


if __name__ == "__main__":

    parser = argparse.ArgumentParser(
        description="use env vars as defaults for argparse"
    )

    parser.add_argument(
        "--alpha", "-a",
        action=env_default('ALPHA'),
        required=False,
        help="""Optional. Value from CLI will be used if present, or from env var ALPHA.
If neither of these is present, value is None becasue no fallback default
was given"""
    )

    parser.add_argument(
        "--beta", "-b",
        action=env_default('BETA'),
        required=True,
        help="""Required. Value from CLI will be used if present, or from env var BETA.
If neither of these is present, it's an error"""
    )

    parser.add_argument(
        "--gamma", "-g",
        action=env_default('GAMMA'),
        required=True,
        default=9001,
        help="""Value from CLI will be used if present, or from env var GAMMA.
If neither of these is present, default of 9001 is used"""
    )

    print parser.parse_args()

EnvDefault.py

"""Provides a utility to inject environment variables into argparse definitions.
Currently requires explicit naming of env vars to check for"""

import argparse
import os

# Courtesy of http://stackoverflow.com/a/10551190 with env-var retrieval fixed
class EnvDefault(argparse.Action):
    """An argparse action class that auto-sets missing default values from env
    vars. Defaults to requiring the argument."""

    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)

# functional sugar for the above
def env_default(envvar):
    def wrapper(**kwargs):
        return EnvDefault(envvar, **kwargs)
    return wrapper