# Configuration Handling

Applications need some kind of configuration. There are different settings you might want to change depending on the application environment like toggling the debug mode, setting the secret key, and other such environment-specific things.

The way Flask is designed usually requires the configuration to be available when the application starts up. You can hard code the configuration in the code, which for many small applications is not actually that bad, but there are better ways.

Independent of how you load your config, there is a config object available which holds the loaded configuration values: The config attribute of the Flask object. This is the place where Flask itself puts certain configuration values and also where extensions can put their configuration values. But this is also where you can have your own configuration.

## Configuration Basics

The config is actually a subclass of a dictionary and can be modified just like any dictionary:

app = Flask(__name__)
app.config['TESTING'] = True


Certain configuration values are also forwarded to the Flask object so you can read and write them from there:

app.testing = True


To update multiple keys at once you can use the dict.update() method:

app.config.update(
TESTING=True,
SECRET_KEY=b'_5#y2L"F4Q8z\n\xec]/'
)


## Environment and Debug Features

The ENV and DEBUG config values are special because they may behave inconsistently if changed after the app has begun setting up. In order to set the environment and debug mode reliably, Flask uses environment variables.

The environment is used to indicate to Flask, extensions, and other programs, like Sentry, what context Flask is running in. It is controlled with the FLASK_ENV environment variable and defaults to production.

Setting FLASK_ENV to development will enable debug mode. flask run will use the interactive debugger and reloader by default in debug mode. To control this separately from the environment, use the FLASK_DEBUG flag.

Changelog

Changed in version 1.0: Added FLASK_ENV to control the environment separately from debug mode. The development environment enables debug mode.

To switch Flask to the development environment and enable debug mode, set FLASK_ENV:

$export FLASK_ENV=development$ flask run


(On Windows, use set instead of export.)

Using the environment variables as described above is recommended. While it is possible to set ENV and DEBUG in your config or code, this is strongly discouraged. They can’t be read early by the flask command, and some systems or extensions may have already configured themselves based on a previous value.

## Builtin Configuration Values

The following configuration values are used internally by Flask:

ENV

What environment the app is running in. Flask and extensions may enable behaviors based on the environment, such as enabling debug mode. The env attribute maps to this config key. This is set by the FLASK_ENV environment variable and may not behave as expected if set in code.

Do not enable development when deploying in production.

Default: 'production'

Changelog

New in version 1.0.

DEBUG

Whether debug mode is enabled. When using flask run to start the development server, an interactive debugger will be shown for unhandled exceptions, and the server will be reloaded when code changes. The debug attribute maps to this config key. This is enabled when ENV is 'development' and is overridden by the FLASK_DEBUG environment variable. It may not behave as expected if set in code.

Do not enable debug mode when deploying in production.

Default: True if ENV is 'development', or False otherwise.

TESTING

Enable testing mode. Exceptions are propagated rather than handled by the the app’s error handlers. Extensions may also change their behavior to facilitate easier testing. You should enable this in your own tests.

Default: False

PROPAGATE_EXCEPTIONS

Exceptions are re-raised rather than being handled by the app’s error handlers. If not set, this is implicitly true if TESTING or DEBUG is enabled.

Default: None

PRESERVE_CONTEXT_ON_EXCEPTION

Don’t pop the request context when an exception occurs. If not set, this is true if DEBUG is true. This allows debuggers to introspect the request data on errors, and should normally not need to be set directly.

Default: None

TRAP_HTTP_EXCEPTIONS

If there is no handler for an HTTPException-type exception, re-raise it to be handled by the interactive debugger instead of returning it as a simple error response.

Default: False

TRAP_BAD_REQUEST_ERRORS

Trying to access a key that doesn’t exist from request dicts like args and form will return a 400 Bad Request error page. Enable this to treat the error as an unhandled exception instead so that you get the interactive debugger. This is a more specific version of TRAP_HTTP_EXCEPTIONS. If unset, it is enabled in debug mode.

Default: None

SECRET_KEY

A secret key that will be used for securely signing the session cookie and can be used for any other security related needs by extensions or your application. It should be a long random string of bytes, although unicode is accepted too. For example, copy the output of this to your config:

$python run-app.py * Running on http://127.0.0.1:5000/ * Restarting with reloader...  On Windows systems use the set builtin instead: > set YOURAPPLICATION_SETTINGS=\path\to\settings.cfg  The configuration files themselves are actual Python files. Only values in uppercase are actually stored in the config object later on. So make sure to use uppercase letters for your config keys. Here is an example of a configuration file: # Example configuration DEBUG = False SECRET_KEY = b'_5#y2L"F4Q8z\n\xec]/'  Make sure to load the configuration very early on, so that extensions have the ability to access the configuration when starting up. There are other methods on the config object as well to load from individual files. For a complete reference, read the Config object’s documentation. ## Configuring from Environment Variables In addition to pointing to configuration files using environment variables, you may find it useful (or necessary) to control your configuration values directly from the environment. Environment variables can be set on Linux or OS X with the export command in the shell before starting the server: $ export SECRET_KEY='5f352379324c22463451387a0aec5d2f'
$export MAIL_ENABLED=false$ python run-app.py
* Running on http://127.0.0.1:5000/


On Windows systems use the set builtin instead:

> set SECRET_KEY='5f352379324c22463451387a0aec5d2f'


While this approach is straightforward to use, it is important to remember that environment variables are strings – they are not automatically deserialized into Python types.

Here is an example of a configuration file that uses environment variables:

import os

_mail_enabled = os.environ.get("MAIL_ENABLED", default="true")
MAIL_ENABLED = _mail_enabled.lower() in {"1", "t", "true"}

SECRET_KEY = os.environ.get("SECRET_KEY")

if not SECRET_KEY:
raise ValueError("No SECRET_KEY set for Flask application")


Notice that any value besides an empty string will be interpreted as a boolean True value in Python, which requires care if an environment explicitly sets values intended to be False.

Make sure to load the configuration very early on, so that extensions have the ability to access the configuration when starting up. There are other methods on the config object as well to load from individual files. For a complete reference, read the Config class documentation.

## Configuration Best Practices

The downside with the approach mentioned earlier is that it makes testing a little harder. There is no single 100% solution for this problem in general, but there are a couple of things you can keep in mind to improve that experience:

1. Create your application in a function and register blueprints on it. That way you can create multiple instances of your application with different configurations attached which makes unit testing a lot easier. You can use this to pass in configuration as needed.
2. Do not write code that needs the configuration at import time. If you limit yourself to request-only accesses to the configuration you can reconfigure the object later on as needed.

## Development / Production

Most applications need more than one configuration. There should be at least separate configurations for the production server and the one used during development. The easiest way to handle this is to use a default configuration that is always loaded and part of the version control, and a separate configuration that overrides the values as necessary as mentioned in the example above:

app = Flask(__name__)
app.config.from_object('yourapplication.default_settings')
app.config.from_envvar('YOURAPPLICATION_SETTINGS')


Then you just have to add a separate config.py file and export YOURAPPLICATION_SETTINGS=/path/to/config.py and you are done. However there are alternative ways as well. For example you could use imports or subclassing.

What is very popular in the Django world is to make the import explicit in the config file by adding from yourapplication.default_settings import * to the top of the file and then overriding the changes by hand. You could also inspect an environment variable like YOURAPPLICATION_MODE and set that to production, development etc and import different hard-coded files based on that.

An interesting pattern is also to use classes and inheritance for configuration:

class Config(object):
DEBUG = False
TESTING = False
DATABASE_URI = 'sqlite:///:memory:'

class ProductionConfig(Config):
DATABASE_URI = 'mysql://[email protected]/foo'

class DevelopmentConfig(Config):
DEBUG = True

class TestingConfig(Config):
TESTING = True


To enable such a config you just have to call into from_object():

app.config.from_object('configmodule.ProductionConfig')


Note that from_object() does not instantiate the class object. If you need to instantiate the class, such as to access a property, then you must do so before calling from_object():

from configmodule import ProductionConfig
app.config.from_object(ProductionConfig())

# Alternatively, import via string:
from werkzeug.utils import import_string
cfg = import_string('configmodule.ProductionConfig')()
app.config.from_object(cfg)


Instantiating the configuration object allows you to use @property in your configuration classes:

class Config(object):
"""Base config, uses staging database server."""
DEBUG = False
TESTING = False
DB_SERVER = '192.168.1.56'

@property
def DATABASE_URI(self):         # Note: all caps
return 'mysql://[email protected]{}/foo'.format(self.DB_SERVER)

class ProductionConfig(Config):
"""Uses production database server."""
DB_SERVER = '192.168.19.32'

class DevelopmentConfig(Config):
DB_SERVER = 'localhost'
DEBUG = True

class TestingConfig(Config):
DB_SERVER = 'localhost'
DEBUG = True
DATABASE_URI = 'sqlite:///:memory:'


There are many different ways and it’s up to you how you want to manage your configuration files. However here a list of good recommendations:

• Keep a default configuration in version control. Either populate the config with this default configuration or import it in your own configuration files before overriding values.
• Use an environment variable to switch between the configurations. This can be done from outside the Python interpreter and makes development and deployment much easier because you can quickly and easily switch between different configs without having to touch the code at all. If you are working often on different projects you can even create your own script for sourcing that activates a virtualenv and exports the development configuration for you.
• Use a tool like fabric in production to push code and configurations separately to the production server(s). For some details about how to do that, head over to the Deploying with Fabric pattern.

## Instance Folders

Changelog

New in version 0.8.

Flask 0.8 introduces instance folders. Flask for a long time made it possible to refer to paths relative to the application’s folder directly (via Flask.root_path). This was also how many developers loaded configurations stored next to the application. Unfortunately however this only works well if applications are not packages in which case the root path refers to the contents of the package.

With Flask 0.8 a new attribute was introduced: Flask.instance_path. It refers to a new concept called the “instance folder”. The instance folder is designed to not be under version control and be deployment specific. It’s the perfect place to drop things that either change at runtime or configuration files.

You can either explicitly provide the path of the instance folder when creating the Flask application or you can let Flask autodetect the instance folder. For explicit configuration use the instance_path parameter:

app = Flask(__name__, instance_path='/path/to/instance/folder')


Please keep in mind that this path must be absolute when provided.

If the instance_path parameter is not provided the following default locations are used:

• Uninstalled module:

/myapp.py
/instance

• Uninstalled package:

/myapp
/__init__.py
/instance

• Installed module or package:

$PREFIX/lib/python2.X/site-packages/myapp$PREFIX/var/myapp-instance


\$PREFIX is the prefix of your Python installation. This can be /usr or the path to your virtualenv. You can print the value of sys.prefix to see what the prefix is set to.

Since the config object provided loading of configuration files from relative filenames we made it possible to change the loading via filenames to be relative to the instance path if wanted. The behavior of relative paths in config files can be flipped between “relative to the application root” (the default) to “relative to instance folder” via the instance_relative_config switch to the application constructor:

app = Flask(__name__, instance_relative_config=True)


Here is a full example of how to configure Flask to preload the config from a module and then override the config from a file in the instance folder if it exists:

app = Flask(__name__, instance_relative_config=True)
app.config.from_object('yourapplication.default_settings')
app.config.from_pyfile('application.cfg', silent=True)


The path to the instance folder can be found via the Flask.instance_path. Flask also provides a shortcut to open a file from the instance folder with Flask.open_instance_resource().

Example usage for both:

filename = os.path.join(app.instance_path, 'application.cfg')
with open(filename) as f: