If you’re building web applications then I hope that you’re following the Twelve-Factor App approach. If so you will be storing your config in the environment and for most of you that means in environment variables.

This approach has a few benefits over storing them in yaml, ini, json, etc. Firstly, it stops you from commiting sensitive information into version control - which it kind of feels weird even saying you should be using if you’re not. Secondly, it allows your configuration to vary substantially across deployments - something it will inevitably have the tendency to do.

Now if you’re following this approach then the question inevitably arises as to how you’re going to execute the app with a given configuration when you run it from the command-line. The following is a simple solution invented with some help from my colleagues at plug.dj (EDIT: Jon pointed out to me we were doing something similiar at SHIFT) - just write a simple shell script for every environment you deploy to like so:

export MY_APP_NAME=superslice
export MY_APP_AWS_KEY=super-secret
export MY_APP_AWS_SECRET_KEY=super-super-secret
# Finally, call whatever command comes after this

For those who don’t know $* is simply the ARGLIST - the list of arguments passed into the application. The usage will make more sense once you see it in action below.

Now we just need to hook our application into a simple system for reading these environment variables. This would look something like the following:

import os

# Configuration variable names as constants

class Config(object):

    def __init__(self, config=None):
        """Initialize config with mapping.
        :param config: (Defaults to os.environ) A dict-like config object.
        :type config: dict
        self.config = config if config else os.environ

    def get(self, key, default=None):
        return self.config.get(key, default)

    def get_bool(self, key, default=False):
        return self.config.get(key, str(default)).lower() in ('true', 't', '1')

Finally, we can grab environment variables and begin using them in our application.

import sys

import config

if __name__ == '__main__':
    cfg = config.Config()
    print 'Welcome to {}'.format(cfg.get(config.APP_NAME))
    print sys.argv

Then when it comes time to run your application simply prepend the shell script.

user@computer:~/app$ ./localsettings.sh bin/app.py --app-arg1 arg1-val --app-arg2
Welcome to superslice
['bin/app.py', '--app-arg1', 'arg1-val', '--app-arg2']