New in version 0.3.
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.
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='...' )
The following configuration values are used internally by Flask:
| enable/disable debug mode |
| enable/disable testing mode |
| explicitly enable or disable the propagation of exceptions. If not set or explicitly set to |
| 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). |
| the secret key |
| the name of the session cookie |
| the domain for the session cookie. If this is not set, the cookie will be valid for all subdomains of |
| the path for the session cookie. If this is not set the cookie will be valid for all of |
| controls if the cookie should be set with the httponly flag. Defaults to |
| controls if the cookie should be set with the secure flag. Defaults to |
| the lifetime of a permanent session as |
| this flag controls how permanent sessions are refreshed. If set to |
| enable/disable x-sendfile |
| the name of the logger |
| the policy of the default logging handler. The default is |
| the name and port number of the server. Required for subdomain support (e.g.: |
| 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 |
| 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. |
| Default cache control max age to use with |
| If this is set to |
| 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 |
| The URL scheme that should be used for URL generation if no URL scheme is available. This defaults to |
| By default Flask serialize object to ascii-encoded JSON. If this is set to |
| By default Flask will serialize JSON objects in a way that the keys are ordered. This is done in order to ensure that independent of the hash seed of the dictionary the return value will be consistent to not trash external HTTP caches. You can override the default behavior by changing this variable. This is not recommended but might give you a performance improvement on the cost of cacheability. |
| If this is set to |
| MIME type used for jsonify responses. |
| Whether to check for modifications of the template source and reload it automatically. By default the value is |
| If this is enabled then every attempt to load a template will write an info message to the logger explaining the attempts to locate the template. This can be useful to figure out why templates cannot be found or wrong templates appear to be loaded. |
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 choose 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.
New in version 0.11: SESSION_REFRESH_EACH_REQUEST
, TEMPLATES_AUTO_RELOAD
, LOGGER_HANDLER_POLICY
, EXPLAIN_TEMPLATE_LOADING
New in version 0.10: JSON_AS_ASCII
, JSON_SORT_KEYS
, JSONIFY_PRETTYPRINT_REGULAR
New in version 0.9: PREFERRED_URL_SCHEME
New in version 0.8: TRAP_BAD_REQUEST_ERRORS
, TRAP_HTTP_EXCEPTIONS
, APPLICATION_ROOT
, SESSION_COOKIE_DOMAIN
, SESSION_COOKIE_PATH
, SESSION_COOKIE_HTTPONLY
, SESSION_COOKIE_SECURE
New in version 0.7: PROPAGATE_EXCEPTIONS
, PRESERVE_CONTEXT_ON_EXCEPTION
New in version 0.6: MAX_CONTENT_LENGTH
New in version 0.5: SERVER_NAME
New in version 0.4: LOGGER_NAME
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 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.
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:
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:
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 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()
© 2007–2020 Pallets
Licensed under the BSD 3-clause License.
https://flask.palletsprojects.com/en/0.12.x/config/