Python Decouple: Strict separation of settings from code

Decouple helps you to organize your settings so that you can change parameters without having to redeploy your app.

It also makes easy for you to:

. store parameters on ini or .env files;

. define comprehensive default values;

. properly convert values to the correct data type;

. have only one configuration module to rule all your instances.

It was originally designed for Django, but became an independent generic tool for separating settings from code.

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Why?

Web framework's settings stores many different kinds of parameters:

Locale and i18n;
Middlewares and Installed Apps;
Resource handles to the database, Memcached, and other backing services;
Credentials to external services such as Amazon S3 or Twitter;
Per-deploy values such as the canonical hostname for the instance.

The first 2 are project settings the last 3 are instance settings.

You should be able to change instance settings without redeploying your app.

Why not just use environment variables?

Envvars works, but since os.environ only returns strings, it's tricky.

Let's say you have an envvar DEBUG=False. If you run:

.. code-block:: python

if os.environ['DEBUG']:
    print True
else:
    print False

It will print True, because os.environ['DEBUG'] returns the string "False". Since it's a non-empty string, it will be evaluated as True.

Decouple provides a solution that doesn't look like a workaround: config('DEBUG', cast=bool).

Install

.. code-block:: console

pip install python-decouple

Usage

On your settings.py.

. Import the `config` object:

.. code-block:: python

 from decouple import config

. Retrieve the configuration parameters:

.. code-block:: python

 SECRET_KEY = config('SECRET_KEY')
 DEBUG = config('DEBUG', default=False, cast=bool)
 EMAIL_HOST = config('EMAIL_HOST', default='localhost')
 EMAIL_PORT = config('EMAIL_PORT', default=25, cast=int)

Where the settings data are stored?

Decouple supports both .ini and .env files.

Ini file ~~~~~~~~

Simply create a settings.ini next to your configuration module in the form:

.. code-block:: ini

[settings]
DEBUG=True
TEMPLATE_DEBUG=%(DEBUG)s
SECRET_KEY=ARANDOMSECRETKEY
DATABASE_URL=mysql://myuser:mypassword@myhost/mydatabase
PERCENTILE=90%%
#COMMENTED=42

Note: Since ConfigParser supports string interpolation, to represent the character % you need to escape it as %%.

Env file ~~~~~~~~

Simply create a .env text file on your repository's root directory in the form:

.. code-block:: console

DEBUG=True
TEMPLATE_DEBUG=True
SECRET_KEY=ARANDOMSECRETKEY
DATABASE_URL=mysql://myuser:mypassword@myhost/mydatabase
PERCENTILE=90%
#COMMENTED=42

Example: How do I use it with Django?

Given that I have a .env file at my repository root directory, here is a snippet of my settings.py.

I also recommend using unipath <https://pypi.python.org/pypi/Unipath>_ and dj-datatabase-url <https://pypi.python.org/pypi/dj-database-url/>_.

.. code-block:: python

# coding: utf-8
from decouple import config
from unipath import Path
from dj_database_url import parse as db_url


BASE_DIR = Path(__file__).parent

DEBUG = config('DEBUG', default=False, cast=bool)
TEMPLATE_DEBUG = DEBUG

DATABASES = {
    'default': config(
        'DATABASE_URL',
        default='sqlite:///' + BASE_DIR.child('db.sqlite3'),
        cast=db_url
    )
}

TIME_ZONE = 'America/Sao_Paulo'
USE_L10N = True
USE_TZ = True

SECRET_KEY = config('SECRET_KEY')

EMAIL_HOST = config('EMAIL_HOST', default='localhost')
EMAIL_PORT = config('EMAIL_PORT', default=25, cast=int)
EMAIL_HOST_PASSWORD = config('EMAIL_HOST_PASSWORD', default='')
EMAIL_HOST_USER = config('EMAIL_HOST_USER', default='')
EMAIL_USE_TLS = config('EMAIL_USE_TLS', default=False, cast=bool)

# ...

Attention with undefined parameters ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

On the above example, all configuration parameters except SECRET_KEY = config('SECRET_KEY') have a default value to fallback if it does not exist on the .env file.

If SECRET_KEY is not present on the .env, decouple will raise an UndefinedValueError.

This fail fast policy helps you avoid chasing misbehaviors when you eventually forget a parameter.

How it works?

Decouple is made of 5 classes:

Config

Coordinates all the configuration retrieval.
RepositoryIni

Can read values from os.environ and ini files, in that order.

Note: Since version 3.0 decouple respects unix precedence of environment variables over config files.
RepositoryEnv

Can read values from os.environ and .env files.

Note: Since version 3.0 decouple respects unix precedence of environment variables over config files.
RepositoryShell

Can only read values from os.environ.
AutoConfig

Detects which configuration repository you're using.

It recursively searches up your configuration module path looking for a settings.ini or a .env file.

The config object is an instance of AutoConfig to improve decouple's usage.

Understanding the CAST argument

By default, all values returned by decouple are strings.

This happens because they are read from text files or the envvars.

However, your Python code may expect some other value type, for example:

Django's DEBUG expects a boolean True or False.
Django's EMAIL_PORT expects an integer.
Django's ALLOWED_HOSTS expects a list of hostnames.

To meet this need, the config function accepts a cast argument which receives any callable, that will be used to transform the string value into something else.

Let's see some examples for the above mentioned cases:

.. code-block:: pycon

>>> os.environ['DEBUG'] = 'False'
>>> config('DEBUG', cast=bool)
False

>>> os.environ['EMAIL_PORT'] = '42'
>>> config('EMAIL_PORT', cast=int)
42

>>> os.environ['ALLOWED_HOSTS'] = '.localhost, .herokuapp.com'
>>> config('ALLOWED_HOSTS', cast=lambda v: [s.strip() for s in v.split(',')])
['.localhost,', '.herokuapp.com']

As you can see, cast is very flexible. But the last example got a bit complex.

Built in Csv Helper

To address the complexity of the last example, Decouple comes with an extensible Csv helper.

Let's improve the last example:

.. code-block:: pycon

>>> os.environ['ALLOWED_HOSTS'] = '.localhost, .herokuapp.com'
>>> config('ALLOWED_HOSTS', cast=Csv())
['.localhost,', '.herokuapp.com']

You can also parametrize the Csv Helper to return other types of data.

.. code-block:: pycon

>>> os.environ['LIST_OF_INTEGERS'] = '1,2,3,4,5'
>>> config('LIST_OF_INTEGERS', cast=Csv(int))
[1, 2, 3, 4, 5]

>>> os.environ['COMPLEX_STRING'] = '%virtual_env%\t *important stuff*\t   trailing spaces   '
>>> csv = Csv(cast=lambda s: s.upper(), delimiter='\t', strip=' %*')
>>> csv('%virtual_env%\t *important stuff*\t   trailing spaces   ')
['VIRTUAL_ENV', 'IMPORTANT STUFF', 'TRAILING SPACES']
"""

License

The MIT License (MIT)

Copyright (c) 2013 Henrique Bastos

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

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