import argparse
import os

"""
Basic example that supplies all arguments via env vars

    export EXAMPLE_POS1=Pos1
    export EXAMPLE_POS2=Pos2
    export EXAMPLE_NAMED1='[val one,val two]'
    export EXAMPLE_NAMED2=
    export
    python3 example.py 
"""

def env_fallback(parser, action):
        if not action.option_strings:
            keys = [ action.metavar ]

        else:
            keys = []
            for arg in action.option_strings:
                if any([arg.startswith(2*c) for c in parser.prefix_chars]):
                    keys.append(arg[2:])

        for key in keys:
            try:
                raw = os.environ['{}_{}'.format(parser.prog, key).upper()]

                # booleans need special handling
                if isinstance(action, argparse._StoreTrueAction):
                    if raw.lower() == 'false':
                        raise argparse.NoFallbackAvailable()
                    else:
                        return []

                if len(raw) > 1 and raw[0] == '[' and raw[-1] == ']':
                    return raw[1:-1].split(',')
                else:
                    return [raw]

            except KeyError:
                pass

        raise argparse.NoFallbackAvailable()

parser = argparse.ArgumentParser(fallback=env_fallback, prog='example')

parser.add_argument('a', metavar='pos1')
parser.add_argument('b', metavar='pos2')
parser.add_argument('--named1', required=True, nargs=2)
parser.add_argument('--named2', action='store_true')

args = parser.parse_args()

print(args)