### argparse

class NoFallbackAvailable(Error):
    pass

class Fallback:

    def __init__(self, parser):
        self.parser = parser
        
    def __call__(self, action):
        raise NoFallbackAvailable()

### typical usecase

import argparse
import os

class EnvFallback(argparse.Fallback):

    def __call__(self, action):
        # potential new attribute for action
        # if action.is_positional:
        if not action.option_strings:
            keys = [ action.metavar ]
        
        else:
            # potential new attribute for action
            # keys = action.long_option_names
            keys = []
            for arg in action.option_strings:
                if any([arg.startswith(2*c) for c in self.parser.prefix_chars]):
                    keys += [arg[2:], arg]
    
        for key in keys:
            try:
                return os.environ['{}_{}'.format(argparse.prog, key).upper()]
            except KeyError:
                pass
        
        raise argparse.NoFallbackAvailable()

parser = argparse.ArgumentParser(fallback=EnvFallback)