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Read config from environment for 12 factor apps

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Python module for reading config from environment for 12-factor apps. Supports Python 2.7 and 3.3+


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?


Install from PyPI:

pip install bienvenue


Bienvenue provides two major functions: unprefix and env. The former is a simple filter to remove a common prefix from the keys in a dictionary. The latter provides a similar interface as os.environ.get, that is a getter with fallback to a default, except that the default is required and provides type hinting for decoding the string value.

Here’s a snippet for your Django

from functools import partial
import os
import bienvenue

env = partial(bienvenue.env, bienvenue.unprefix('DJANGO_', os.environ))

Now environment settings can be extracted as follows:

DEBUG = env('DEBUG', True)
SECRET_KEY = env('SECRET_KEY', 'fallback-secret-for-dev')

In this example, DEBUG will be extracted from the environment variable DJANGO_DEBUG (because of calling bienvenue.unprefix above) and then interpreted as a boolean value, honoring common strings such as on/off, true/false, and yes/no.

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

ALLOWED_HOSTS will be extracted from DJANGO_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.

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