Setting options from environment variables when using argparse

Question

I have a script which has certain options that can either be passed on the command line, or from environment variables. The CLI should take precedence if both are present, a

Answer 1

The topic is quite old, but I had similar problem and I thought I would share my solution with you. Unfortunately custom action solution suggested by @Russell Heilling doesn't work for me for couple of reasons:

• It prevents me from using predefined actions (like store_true)
• I would rather like it to fallback to default when envvar is not in os.environ (that could be easily fixed)
• I would like to have this behaviour for all of my arguments without specifying action or envvar (which should always be action.dest.upper())

Here's my solution (in Python 3):

class CustomArgumentParser(argparse.ArgumentParser):
    class _CustomHelpFormatter(argparse.ArgumentDefaultsHelpFormatter):
        def _get_help_string(self, action):
            help = super()._get_help_string(action)
            if action.dest != 'help':
                help += ' [env: {}]'.format(action.dest.upper())
            return help

    def __init__(self, *, formatter_class=_CustomHelpFormatter, **kwargs):
        super().__init__(formatter_class=formatter_class, **kwargs)

    def _add_action(self, action):
        action.default = os.environ.get(action.dest.upper(), action.default)
        return super()._add_action(action)