1   Configuration Handling¶

1.1   Configuration Basics¶

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

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

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

app.debug = True

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

app.config.update(
    DEBUG=True,
    SECRET_KEY='...'
)

1.2   Builtin Configuration Values¶

The following configuration values are used internally by Flask:

1.3   Configuring from Files¶

Configuration becomes more useful if you can store it in a separate file, ideally located outside the actual application package. This makes packaging and distributing your application possible via various package handling tools (Deploying with Setuptools) and finally modifying the configuration file afterwards.

So a common pattern is this:

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

This first loads the configuration from the yourapplication.default_settings module and then overrides the values with the contents of the file the :envvar:YOURAPPLICATION_SETTINGS environment variable points to. This environment variable can be set on Linux or OS X with the export command in the shell before starting the server:

$ export YOURAPPLICATION_SETTINGS=/path/to/settings.cfg
$ 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 = '?\xbf,\xb4\x8d\xa3"<\x9c\xb0@\x0f5\xab,w\xee\x8d$0\x13\x8b83'

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.

1.4   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 unittesting 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.

1.5   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 hardcoded 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://user@localhost/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')

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.

1.6   Instance Folders¶

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 config 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:
    config = f.read()

# or via open_instance_resource:
with app.open_instance_resource('application.cfg') as f:
    config = f.read()