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Configuration Handling

Applications need some kind of configuration. There are different things you might want to change like toggling debug mode, the secret key, and a lot of very similar things.

The way Flask is designed usually requires the configuration to be available when the application starts up. You can hardcode 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 :attr:`~flask.Flask.config` attribute of the :class:`~flask.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 :attr:`~flask.Flask.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 :attr:`~flask.Flask` object so that you can read and write them from there:

app.debug = True

To update multiple keys at once you can use the :meth:`dict.update` method:

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

Builtin Configuration Values

The following configuration values are used internally by Flask:

DEBUG enable/disable debug mode
TESTING enable/disable testing mode
PROPAGATE_EXCEPTIONS explicitly enable or disable the propagation of exceptions. If not set or explicitly set to None this is implicitly true if either TESTING or DEBUG is true.
PRESERVE_CONTEXT_ON_EXCEPTION By default if the application is in debug mode the request context is not popped on exceptions to enable debuggers to introspect the data. This can be disabled by this key. You can also use this setting to force-enable it for non debug execution which might be useful to debug production applications (but also very risky).
SECRET_KEY the secret key
SESSION_COOKIE_NAME the name of the session cookie
PERMANENT_SESSION_LIFETIME the lifetime of a permanent session as :class:`datetime.timedelta` object.
USE_X_SENDFILE enable/disable x-sendfile
LOGGER_NAME the name of the logger
SERVER_NAME the name and port number of the server. Required for subdomain support (e.g.: 'localhost:5000')
MAX_CONTENT_LENGTH If set to a value in bytes, Flask will reject incoming requests with a content length greater than this by returning a 413 status code.

More on SERVER_NAME

The SERVER_NAME key is used for the subdomain support. Because Flask cannot guess the subdomain part without the knowledge of the actual server name, this is required if you want to work with subdomains. This is also used for the session cookie.

Please keep in mind that not only Flask has the problem of not knowing what subdomains are, your web browser does as well. Most modern web browsers will not allow cross-subdomain cookies to be set on a server name without dots in it. So if your server name is 'localhost' you will not be able to set a cookie for 'localhost' and every subdomain of it. Please chose a different server name in that case, like 'myapplication.local' and add this name + the subdomains you want to use into your host config or setup a local bind.

Configuring from Files

Configuration becomes more useful if you can configure from a file, and ideally that file would be outside of the actual application package so that you can install the package with distribute (:ref:`distribute-deployment`) and still modify that 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 configuration file:

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 :class:`~flask.Config` object's documentation.

Configuration Best Practices

The downside with the approach mentioned earlier is that it makes testing a little harder. There is no one 100% solution for this problem in general, but there are a couple of things you can do 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.

Development / Production

Most applications need more than one configuration. There will at least be a separate configuration for a production server and one used during development. The easiest way to handle this is to use a default configuration that is always loaded and part of 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 an 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 :meth:`~flask.Config.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 :ref:`fabric-deployment` pattern.
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