python-decouple 2.3

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:

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

pip install python-decouple

Usage

On your settings.py.

1. Import the config object:

   from decouple import config

2. Retrieve the configuration parameters:

   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.

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

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

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 and dj-datatabase-url.

# 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)

# ...

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:

>>> 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:

>>> 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.

>>> os.environ['LIST_OF_INTEGERS'] = '1,2,3,4,5'
>>> config('ALLOWED_HOSTS', 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']
"""