Rationale

In a 12-factor app, config comes from the environment. For example, on Heroku, config variables are set using heroku config:set on the command line, then passed to the application in the environment.

Reading and parsing config from the environment is tedious and error-prone. For example, a Django app has settings.DEBUG which should be either True or False. The operator runs heroku config:set DJANGO_DEBUG off intending to disable it, but the application only sees a non-empty string and treats the value as True.

This sort of problem can be handled case-by-case but doesn’t need to be. Clearly a configuration setting with a fallback boolean value should be interpreted as boolean from the environment. Likewise a fallback integer setting indicates the associated environment variable should be converted from string to int.

Sounds like a job for a small and well-tested Python module, right?

Installation

Install from PyPI:

pip install bienvenue

Usage

Bienvenue provides the function make_env_reader which returns a new function for accessing the environment. For example:

from bienvenue import make_env_reader

env = make_env_reader(prefix='MY_')

DEBUG = env('DEBUG', False)
SECRET_KEY = env('SECRET_KEY', 'fallback-secret-for-dev')
ALLOWED_HOSTS = env('ALLOWED_HOSTS', [])

The env reader looks for MY_DEBUG in os.environ. If it’s not there, the default value False will be returned. However if it’s found, then the value there will be interpreted according to the type of the default, in this case boolean. Common strings such as on/off, true/false and yes/no are interpreted into True or False.

Likewise SECRET_KEY will be extracted from MY_SECRET_KEY and then interpreted as a string value.

ALLOWED_HOSTS will be extracted from MY_ALLOWED_HOSTS and JSON decoded as a list, since the default value is a list.

If bienvenue encounters unknown types or values during parsing, it will log an error and fall back to the provided default.

Legal

Copyright 2017 Scampersand LLC

Released under the MIT license