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, and an error occur if neither are set.

I could check that the option is assigned after parsing, but I prefer to let argparse to do the heavy lifting and be responsible for displaying the usage statement if parsing fails.

I have come up with a couple of alternative approaches to this (which I will post below as answers so they can be discussed separately) but they feel pretty kludgey to me and I think that I am missing something.

Is there an accepted "best" way of doing this?

(Edit to make the desired behaviour clear when both the CLI option and environment variable are unset)

Answer 1

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()) 

Answer 2

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()