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 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['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:


Debug Mode with the flask Script

If you use the flask script to start a local development server, to enable the debug mode, you need to export the FLASK_DEBUG environment variable before running the server:

$ export FLASK_DEBUG=1
$ flask run

(On Windows you need to use set instead of export).

app.debug and app.config['DEBUG'] are not compatible with the flask script. They only worked when using method.

Builtin Configuration Values

The following configuration values are used internally by Flask:


Enable debug mode. When using the development server with flask run or, an interactive debugger will be shown for unhanlded exceptions, and the server will be reloaded when code changes.

Do not enable debug mode in production.

Default: False


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


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


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


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


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


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 -c 'import os; print(os.urandom(16))'

Do not reveal the secret key when posting questions or committing code.

Default: None

The name of the session cookie. Can be changed in case you already have a cookie with the same name.

Default: 'session'

The domain match rule that the session cookie will be valid for. If not set, the cookie will be valid for all subdomains of SERVER_NAME. If False, the cookie’s domain will not be set.

Default: None

The path that the session cookie will be valid for. If not set, the cookie will be valid underneath APPLICATION_ROOT or / if that is not set.

Default: None

Browsers will not allow JavaScript access to cookies marked as “HTTP only” for security.

Default: True

Browsers will only send cookies with requests over HTTPS if the cookie is marked “secure”. The application must be served over HTTPS for this to make sense.

Default: False


If session.permanent is true, the cookie’s max age will be set to this number of seconds. Can either be a datetime.timedelta or an int.

Default: timedelta(days=31) (2678400 seconds)


Control whether the cookie is sent with every response when session.permanent is true. Sending the cookie every time (the default) can more reliably keep the session from expiring, but uses more bandwidth. Non-permanent sessions are not affected.

Default: True


When serving files, set the X-Sendfile header instead of serving the data with Flask. Some web servers, such as Apache, recognize this and serve the data more efficiently. This only makes sense when using such a server.

Default: False


When serving files, set the cache control max age to this number of seconds. Can either be a datetime.timedelta or an int. Override this value on a per-file basis using get_send_file_max_age() on the application or blueprint.

Default: timedelta(hours=12) (43200 seconds)


The name of the logger that the Flask application sets up. If not set, it will take the import name passed to Flask.__init__.

Default: None


When to activate the application’s logger handler. 'always' always enables it, 'debug' only activates it in debug mode, 'production' only activates it when not in debug mode, and 'never' never enables it.

Default: 'always'


Inform the application what host and port it is bound to. Required for subdomain route matching support.

If set, will be used for the session cookie domain if SESSION_COOKIE_DOMAIN is not set. Modern web browsers will not allow setting cookies for domains without a dot. To use a domain locally, add any names that should route to the app to your hosts file.

If set, url_for can generate external URLs with only an application context instead of a request context.

Default: None


Inform the application what path it is mounted under by the application / web server.

Will be used for the session cookie path if SESSION_COOKIE_PATH is not set.

Default: '/'


Use this scheme for generating external URLs when not in a request context.

Default: 'http'


Don’t read more than this many bytes from the incoming request data. If not set and the request does not specify a CONTENT_LENGTH, no data will be read for security.

Default: None


Serialize objects to ASCII-encoded JSON. If this is disabled, the JSON will be returned as a Unicode string, or encoded as UTF-8 by jsonify. This has security implications when rendering the JSON in to JavaScript in templates, and should typically remain enabled.

Default: True


Sort the keys of JSON objects alphabetically. This is useful for caching because it ensures the data is serialized the same way no matter what Python’s hash seed is. While not recommended, you can disable this for a possible performance improvement at the cost of caching.

Default: True


jsonify responses will be output with newlines, spaces, and indentation for easier reading by humans. Always enabled in debug mode.

Default: False


The mimetype of jsonify responses.

Default: 'application/json'


Reload templates when they are changed. If not set, it will be enabled in debug mode.

Default: None


Log debugging information tracing how a template file was loaded. This can be useful to figure out why a template was not loaded or the wrong file appears to be loaded.

Default: False

New in version 0.4: LOGGER_NAME

New in version 0.5: SERVER_NAME

New in version 0.6: MAX_CONTENT_LENGTH



New in version 0.9: PREFERRED_URL_SCHEME



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__)

This first loads the configuration from the yourapplication.default_settings module and then overrides the values with the contents of the file the 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
 * Running on
 * 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 DEBUG=False
$ python
 * Running on
 * Restarting with reloader...

On Windows systems use the set builtin instead:

>set SECRET_KEY='5f352379324c22463451387a0aec5d2f'
>set DEBUG=False

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:

# Example configuration
import os

ENVIRONMENT_DEBUG = os.environ.get("DEBUG", default=False)
if ENVIRONMENT_DEBUG.lower() in ("f", "false"):

SECRET_KEY = os.environ.get("SECRET_KEY", default=None)
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 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__)

Then you just have to add a separate file and export YOURAPPLICATION_SETTINGS=/path/to/ 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():


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

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:

  • Uninstalled package:

  • Installed module or package:


    $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_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 =

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