Everett

Everett is a Python configuration library for your app.

Code:	https://github.com/willkg/everett
Issues:	https://github.com/willkg/everett/issues
License:	MPL v2
Documentation:	https://everett.readthedocs.io/

Goals

Goals of Everett:

1. flexible configuration from multiple configured environments
2. easy testing with configuration
3. easy documentation of configuration for users

From that, Everett has the following features:

• is composeable and flexible
• makes it easier to provide helpful error messages for users trying to configure your software
• supports auto-documentation of configuration with a Sphinx autocomponent directive
• has an API for testing configuration variations in your tests
• can pull configuration from a variety of specified sources (environment, INI files, YAML files, dict, write-your-own)
• supports parsing values (bool, int, lists of things, classes, write-your-own)
• supports key namespaces
• supports component architectures
• works with whatever you're writing--command line tools, web sites, system daemons, etc

Everett is inspired by python-decouple and configman.

Quick start

Fast start example

You're writing an app and want it to look for configuration:

1. in an .env file in the current working directory
2. then in the process environment

You set up a configuration manager like this:

from everett.manager import ConfigManager

config = ConfigManager.basic_config()

and use it to get configuration values like this:

api_host = config("api_host")
max_bytes = config("max_bytes", default="1000", parser=int)
debug_mode = config("debug", default="False", parser=bool)

You can create a basic configuration that points to your documentation like this:

config = ConfigManager.basic_config(
    doc="Check https://example.com/configuration for docs."
)

If the user sets DEBUG with a bad value, they get a helpful error message with the documentation for the configuration option and the ConfigManager:

$ DEBUG=foo python myprogram.py
<traceback>
DEBUG requires a value parseable by bool
DEBUG docs: Set to True for debugmode; False for regular mode.
Project docs: Check https://example.com/configuration for docs.

You can use config_override in your tests to test various configuration values:

from everett.manager import config_override

from myapp import debug_mode


def test_debug_on():
    with config_override(DEBUG="on"):
        assert debug_mode == True

def test_debug_off():
    with config_override(DEBUG="off"):
        assert debug_mode == False

When you outgrow that or need different variations of it, you can switch to creating a ConfigManager instance that meets your needs.

More complex example

You're writing an app and want to pull configuration (in order of prcedence):

1. from the process environment
2. from an INI file stored in a place specified by (in order of precedence):
   1. MYAPP_INI in the environment
   2. ~/.myapp.ini
   3. /etc/myapp.ini

First, you need need to install the additional requirements for INI file environments:

pip install 'everett[ini]'

Then set up the ConfigManager:

# myapp.py

import os
import sys

from everett.ext.inifile import ConfigIniEnv
from everett.manager import ConfigManager, ConfigOSEnv


CONFIG = ConfigManager(
    # Specify one or more configuration environments in
    # the order they should be checked
    environments=[
        # Look in OS process environment first
        ConfigOSEnv(),

        # Look in INI files in order specified
        ConfigIniEnv(
            possible_paths=[
                os.environ.get("MYAPP_INI"),
                "~/.myapp.ini",
                "/etc/myapp.ini"
            ]
        ),
    ],

    # Provide users a link to documentation for when they hit
    # configuration errors
    doc="Check https://example.com/configuration for docs."
)

Then use it:

# myapp.py continued

def is_debug(config):
    return config(
        "debug",
        default="False",
        parser=bool,
        doc="Set to True for debugmode; False for regular mode."
    )

if is_debug(CONFIG):
    print('DEBUG MODE ON!')

Let's write some tests that verify behavior based on the debug configuration value:

from myapp import CONFIG, is_debug

from everett.manager import config_override


@config_override(DEBUG="true")
def test_debug_true():
    assert is_debug(CONFIG) is True


def test_debug_false():
    with config_override(DEBUG="false"):
        assert is_debug(CONFIG) is False

If the user sets DEBUG with a bad value, they get a helpful error message with the documentation for the configuration option and the ConfigManager:

$ DEBUG=foo python myprogram.py
<traceback>
DEBUG requires a value parseable by bool
DEBUG docs: Set to True for debugmode; False for regular mode.
Project docs: Check https://example.com/configuration for docs.

Configuration classes

Everett supports centralizing your configuration in a class. Instead of having configuration-related bits defined across your codebase, you can define it in a class. Let's rewrite the above example using a configuration class.

First, create a configuration class:

# myapp.py

import os
import sys

from everett.ext.inifile import ConfigIniEnv
from everett.manager import ConfigManager, ConfigOSEnv, Option


class AppConfig:
    class Config:
        debug = Option(
            parser=bool,
            default="false",
            doc="Switch debug mode on and off.")
        )

Then we set up a ConfigManager to look at the process environment for configuration and bound to the configuration options specified in AppConfig:

# myapp.py continued

def get_config():
    manager = ConfigManager(
        # Specify environments to check for configuration
        environments=[
            ConfigOSEnv(),
        ],

        # Provide users a link to documentation for when they hit
        # configuration errors
        doc="Check https://example.com/configuration for docs."
    )

    # Apply the configuration class to the configuration manager
    # so that it handles option properties like defaults, parsers,
    # documentation, and so on.
    return manager.with_options(AppConfig())

Then use it:

# myapp.py continued

config = get_config()

if config("debug"):
    print("DEBUG MODE ON!")

Further, you can auto-generate configuration documentation by including the everett.sphinxext Sphinx extension and using the autocomponent directive:

.. autocomponent:: path.to.AppConfig

That has some niceties:

1. your application configuration is centralized in one place instead of spread out across your code base
2. you can use the autocomponent Sphinx directive to auto-generate configuration documentation for your users

Everett components

Everett supports components that require configuration. Say your app needs to connect to RabbitMQ. With Everett, you can define the component's configuration needs in the component class:

from everett.manager import Option


class RabbitMQComponent:
    class Config:
        host = Option(doc="RabbitMQ host to connect to")
        port = Option(default="5672", doc="Port to use", parser=int)
        queue_name = Option(doc="Queue to insert things into")

    def __init__(self, config):
        # Bind the configuration to just the configuration this
        # component requires such that this component is
        # self-contained
        self.config = config.with_options(self)

        self.host = self.config("host")
        self.port = self.config("port")
        self.queue_name = self.config("queue_name")

Then you could instantiate a RabbitMQComponent that looks for configuration keys in the rmq namespace:

queue = RabbitMQComponent(config.with_namespace("rmq"))

The RabbitMQComponent has a HOST key, so your configuration would need to define RMQ_HOST.

You can auto-generate configuration documentation for this component in your Sphinx docs by including the everett.sphinxext Sphinx extension and using the autocomponent directive:

.. autocomponent:: path.to.RabbitMQComponent
   :namespace: rmq

Say your app actually needs to connect to two separate queues--one for regular processing and one for priority processing:

from everett.manager import ConfigManager

config = ConfigManager.basic_config()

# Apply the "rmq" namespace to the configuration so all keys are
# prepended with RMQ_
rmq_config = config.with_namespace("rmq")

# Create a RabbitMQComponent with RMQ_REGULAR_ prepended to keys
regular_queue = RabbitMQComponent(rmq_config.with_namespace("regular"))

# Create a RabbitMQComponent with RMQ_PRIORITY_ prepended to keys
priority_queue = RabbitMQComponent(rmq_config.with_namespace("priority"))

In your environment, you provide the regular queue configuration with RMQ_REGULAR_HOST, etc and the priority queue configuration with RMQ_PRIORITY_HOST, etc.

Same component code. Two different instances pulling configuration from two different namespaces.

Components support subclassing, mixins and all that, too.

Install

Install from PyPI

Run:

$ pip install everett

If you want to use the ConfigIniEnv, you need to install its requirements as well:

$ pip install 'everett[ini]'

If you want to use the ConfigYamlEnv, you need to install its requirements as well:

$ pip install 'everett[yaml]'

Install for hacking

Run:

# Clone the repository
$ git clone https://github.com/willkg/everett

# Create a virtualenvironment
$ mkvirtualenv --python /usr/bin/python3 everett

# Install Everett and dev requirements
$ pip install -e '.[dev,ini,yaml]'

Why not other libs?

Most other libraries I looked at had one or more of the following issues:

• were tied to a specific web app framework
• didn't allow you to specify configuration sources
• provided poor error messages when users configure things wrong
• had a global configuration object
• made it really hard to override specific configuration when writing tests
• had no facilities for auto-generating configuration documentation