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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 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 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
SESSION_COOKIE_DOMAIN the domain for the session cookie. If this is not set, the cookie will be valid for all subdomains of SERVER_NAME.
SESSION_COOKIE_PATH the path for the session cookie. If this is not set the cookie will be valid for all of APPLICATION_ROOT or if that is not set for '/'.
SESSION_COOKIE_HTTPONLY controls if the cookie should be set with the httponly flag. Defaults to True.
SESSION_COOKIE_SECURE controls if the cookie should be set with the secure flag. Defaults to False.
PERMANENT_SESSION_LIFETIME the lifetime of a permanent session as :class:`datetime.timedelta` object. Starting with Flask 0.8 this can also be an integer representing seconds.
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.: 'myapp.dev:5000') Note that localhost does not support subdomains so setting this to “localhost” does not help. Setting a SERVER_NAME also by default enables URL generation without a request context but with an application context.
APPLICATION_ROOT If the application does not occupy a whole domain or subdomain this can be set to the path where the application is configured to live. This is for session cookie as path value. If domains are used, this should be None.
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.
SEND_FILE_MAX_AGE_DEFAULT: Default cache control max age to use with :meth:`flask.Flask.send_static_file`, in seconds. Override this value on a per-file basis using the :meth:`flask.Flask.get_send_file_options` and :meth:`flask.Blueprint.get_send_file_options` hooks. Defaults to 43200 (12 hours).
TRAP_HTTP_EXCEPTIONS If this is set to True Flask will not execute the error handlers of HTTP exceptions but instead treat the exception like any other and bubble it through the exception stack. This is helpful for hairy debugging situations where you have to find out where an HTTP exception is coming from.
TRAP_BAD_REQUEST_ERRORS Werkzeug's internal data structures that deal with request specific data will raise special key errors that are also bad request exceptions. Likewise many operations can implicitly fail with a BadRequest exception for consistency. Since it's nice for debugging to know why exactly it failed this flag can be used to debug those situations. If this config is set to True you will get a regular traceback instead.
PREFERRED_URL_SCHEME The URL scheme that should be used for URL generation if no URL scheme is available. This defaults to http.

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 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 (:ref:`distribute-deployment`) 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 :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 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.

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 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.

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 :attr:`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: :attr:`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 :attr:`Flask.instance_path`. Flask also provides a shortcut to open a file from the instance folder with :meth:`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()
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