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 hard code 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.
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['TESTING'] = True
Certain configuration values are also forwarded to the :attr:`~flask.Flask` object so you can read and write them from there:
app.testing = True
To update multiple keys at once you can use the :meth:`dict.update` method:
app.config.update(
TESTING=True,
SECRET_KEY='192b9bdd22ab9ed4d12e236c78afcb9a393ec15f71bbf5dc987d54727823bcbf'
)
The :data:`DEBUG` config value is special because it may behave inconsistently if
changed after the app has begun setting up. In order to set debug mode reliably, use the
--debug option on the flask or flask run command. flask run will use the
interactive debugger and reloader by default in debug mode.
$ flask --app hello run --debug
Using the option is recommended. While it is possible to set :data:`DEBUG` in your
config or code, this is strongly discouraged. It can't be read early by the
flask run command, and some systems or extensions may have already configured
themselves based on a previous value.
The following configuration values are used internally by Flask:
.. py:data:: DEBUG
Whether debug mode is enabled. When using ``flask run`` to start the development
server, an interactive debugger will be shown for unhandled exceptions, and the
server will be reloaded when code changes. The :attr:`~flask.Flask.debug` attribute
maps to this config key. This is set with the ``FLASK_DEBUG`` environment variable.
It may not behave as expected if set in code.
**Do not enable debug mode when deploying in production.**
Default: ``False``
.. py:data:: TESTING
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``
.. py:data:: PROPAGATE_EXCEPTIONS
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``
.. py:data:: TRAP_HTTP_EXCEPTIONS
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``
.. py:data:: TRAP_BAD_REQUEST_ERRORS
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``
.. py:data:: SECRET_KEY
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 ``bytes`` or ``str``. For
example, copy the output of this to your config::
$ python -c 'import secrets; print(secrets.token_hex())'
'192b9bdd22ab9ed4d12e236c78afcb9a393ec15f71bbf5dc987d54727823bcbf'
**Do not reveal the secret key when posting questions or committing code.**
Default: ``None``
.. py:data:: SESSION_COOKIE_NAME
The name of the session cookie. Can be changed in case you already have a
cookie with the same name.
Default: ``'session'``
.. py:data:: SESSION_COOKIE_DOMAIN
The value of the ``Domain`` parameter on the session cookie. If not set, browsers
will only send the cookie to the exact domain it was set from. Otherwise, they
will send it to any subdomain of the given value as well.
Not setting this value is more restricted and secure than setting it.
Default: ``None``
.. versionchanged:: 2.3
Not set by default, does not fall back to ``SERVER_NAME``.
.. py:data:: SESSION_COOKIE_PATH
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``
.. py:data:: SESSION_COOKIE_HTTPONLY
Browsers will not allow JavaScript access to cookies marked as "HTTP only"
for security.
Default: ``True``
.. py:data:: SESSION_COOKIE_SECURE
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``
.. py:data:: SESSION_COOKIE_SAMESITE
Restrict how cookies are sent with requests from external sites. Can
be set to ``'Lax'`` (recommended) or ``'Strict'``.
See :ref:`security-cookie`.
Default: ``None``
.. versionadded:: 1.0
.. py:data:: PERMANENT_SESSION_LIFETIME
If ``session.permanent`` is true, the cookie's expiration will be set this
number of seconds in the future. Can either be a
:class:`datetime.timedelta` or an ``int``.
Flask's default cookie implementation validates that the cryptographic
signature is not older than this value.
Default: ``timedelta(days=31)`` (``2678400`` seconds)
.. py:data:: SESSION_REFRESH_EACH_REQUEST
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``
.. py:data:: USE_X_SENDFILE
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``
.. py:data:: SEND_FILE_MAX_AGE_DEFAULT
When serving files, set the cache control max age to this number of
seconds. Can be a :class:`datetime.timedelta` or an ``int``.
Override this value on a per-file basis using
:meth:`~flask.Flask.get_send_file_max_age` on the application or
blueprint.
If ``None``, ``send_file`` tells the browser to use conditional
requests will be used instead of a timed cache, which is usually
preferable.
Default: ``None``
.. py:data:: SERVER_NAME
Inform the application what host and port it is bound to. Required
for subdomain route matching support.
If set, ``url_for`` can generate external URLs with only an application
context instead of a request context.
Default: ``None``
.. versionchanged:: 2.3
Does not affect ``SESSION_COOKIE_DOMAIN``.
.. py:data:: APPLICATION_ROOT
Inform the application what path it is mounted under by the application /
web server. This is used for generating URLs outside the context of a
request (inside a request, the dispatcher is responsible for setting
``SCRIPT_NAME`` instead; see :doc:`/patterns/appdispatch`
for examples of dispatch configuration).
Will be used for the session cookie path if ``SESSION_COOKIE_PATH`` is not
set.
Default: ``'/'``
.. py:data:: PREFERRED_URL_SCHEME
Use this scheme for generating external URLs when not in a request context.
Default: ``'http'``
.. py:data:: MAX_CONTENT_LENGTH
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``
.. py:data:: TEMPLATES_AUTO_RELOAD
Reload templates when they are changed. If not set, it will be enabled in
debug mode.
Default: ``None``
.. py:data:: EXPLAIN_TEMPLATE_LOADING
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``
.. py:data:: MAX_COOKIE_SIZE
Warn if cookie headers are larger than this many bytes. Defaults to
``4093``. Larger cookies may be silently ignored by browsers. Set to
``0`` to disable the warning.
.. versionadded:: 0.4 ``LOGGER_NAME``
.. versionadded:: 0.5 ``SERVER_NAME``
.. versionadded:: 0.6 ``MAX_CONTENT_LENGTH``
.. versionadded:: 0.7 ``PROPAGATE_EXCEPTIONS``, ``PRESERVE_CONTEXT_ON_EXCEPTION``
.. versionadded:: 0.8 ``TRAP_BAD_REQUEST_ERRORS``, ``TRAP_HTTP_EXCEPTIONS``, ``APPLICATION_ROOT``, ``SESSION_COOKIE_DOMAIN``, ``SESSION_COOKIE_PATH``, ``SESSION_COOKIE_HTTPONLY``, ``SESSION_COOKIE_SECURE``
.. versionadded:: 0.9 ``PREFERRED_URL_SCHEME``
.. versionadded:: 0.10 ``JSON_AS_ASCII``, ``JSON_SORT_KEYS``, ``JSONIFY_PRETTYPRINT_REGULAR``
.. versionadded:: 0.11 ``SESSION_REFRESH_EACH_REQUEST``, ``TEMPLATES_AUTO_RELOAD``, ``LOGGER_HANDLER_POLICY``, ``EXPLAIN_TEMPLATE_LOADING``
.. versionchanged:: 1.0
``LOGGER_NAME`` and ``LOGGER_HANDLER_POLICY`` were removed. See
:doc:`/logging` for information about configuration.
Added :data:`ENV` to reflect the :envvar:`FLASK_ENV` environment
variable.
Added :data:`SESSION_COOKIE_SAMESITE` to control the session
cookie's ``SameSite`` option.
Added :data:`MAX_COOKIE_SIZE` to control a warning from Werkzeug.
.. versionchanged:: 2.2
Removed ``PRESERVE_CONTEXT_ON_EXCEPTION``.
.. versionchanged:: 2.3
``JSON_AS_ASCII``, ``JSON_SORT_KEYS``, ``JSONIFY_MIMETYPE``, and
``JSONIFY_PRETTYPRINT_REGULAR`` were removed. The default ``app.json`` provider has
equivalent attributes instead.
.. versionchanged:: 2.3
``ENV`` was removed.
Configuration becomes more useful if you can store it in a separate file, ideally located outside the actual application package. You can deploy your application, then separately configure it for the specific deployment.
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 in the shell before starting the server:
.. tabs::
.. group-tab:: Bash
.. code-block:: text
$ export YOURAPPLICATION_SETTINGS=/path/to/settings.cfg
$ flask run
* Running on http://127.0.0.1:5000/
.. group-tab:: Fish
.. code-block:: text
$ set -x YOURAPPLICATION_SETTINGS /path/to/settings.cfg
$ flask run
* Running on http://127.0.0.1:5000/
.. group-tab:: CMD
.. code-block:: text
> set YOURAPPLICATION_SETTINGS=\path\to\settings.cfg
> flask run
* Running on http://127.0.0.1:5000/
.. group-tab:: Powershell
.. code-block:: text
> $env:YOURAPPLICATION_SETTINGS = "\path\to\settings.cfg"
> flask run
* Running on http://127.0.0.1:5000/
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 SECRET_KEY = '192b9bdd22ab9ed4d12e236c78afcb9a393ec15f71bbf5dc987d54727823bcbf'
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.
It is also possible to load configuration from a file in a format of your choice using :meth:`~flask.Config.from_file`. For example to load from a TOML file:
import tomllib
app.config.from_file("config.toml", load=tomllib.load, text=False)Or from a JSON file:
import json
app.config.from_file("config.json", load=json.load)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. Flask can be instructed to load all environment variables starting with a specific prefix into the config using :meth:`~flask.Config.from_prefixed_env`.
Environment variables can be set in the shell before starting the server:
.. tabs::
.. group-tab:: Bash
.. code-block:: text
$ export FLASK_SECRET_KEY="5f352379324c22463451387a0aec5d2f"
$ export FLASK_MAIL_ENABLED=false
$ flask run
* Running on http://127.0.0.1:5000/
.. group-tab:: Fish
.. code-block:: text
$ set -x FLASK_SECRET_KEY "5f352379324c22463451387a0aec5d2f"
$ set -x FLASK_MAIL_ENABLED false
$ flask run
* Running on http://127.0.0.1:5000/
.. group-tab:: CMD
.. code-block:: text
> set FLASK_SECRET_KEY="5f352379324c22463451387a0aec5d2f"
> set FLASK_MAIL_ENABLED=false
> flask run
* Running on http://127.0.0.1:5000/
.. group-tab:: Powershell
.. code-block:: text
> $env:FLASK_SECRET_KEY = "5f352379324c22463451387a0aec5d2f"
> $env:FLASK_MAIL_ENABLED = "false"
> flask run
* Running on http://127.0.0.1:5000/
The variables can then be loaded and accessed via the config with a key equal to the environment variable name without the prefix i.e.
app.config.from_prefixed_env()
app.config["SECRET_KEY"] # Is "5f352379324c22463451387a0aec5d2f"The prefix is FLASK_ by default. This is configurable via the
prefix argument of :meth:`~flask.Config.from_prefixed_env`.
Values will be parsed to attempt to convert them to a more specific type
than strings. By default :func:`json.loads` is used, so any valid JSON
value is possible, including lists and dicts. This is configurable via
the loads argument of :meth:`~flask.Config.from_prefixed_env`.
When adding a boolean value with the default JSON parsing, only "true"
and "false", lowercase, are valid values. Keep in mind that any
non-empty string is considered True by Python.
It is possible to set keys in nested dictionaries by separating the
keys with double underscore (__). Any intermediate keys that don't
exist on the parent dict will be initialized to an empty dict.
$ export FLASK_MYAPI__credentials__username=user123
app.config["MYAPI"]["credentials"]["username"] # Is "user123"On Windows, environment variable keys are always uppercase, therefore
the above example would end up as MYAPI__CREDENTIALS__USERNAME.
For even more config loading features, including merging and case-insensitive Windows support, try a dedicated library such as Dynaconf, which includes integration with Flask.
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:
- 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 unit testing a lot easier. You can use this to pass in configuration as needed.
- 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.
- Make sure to load the configuration very early on, so that
extensions can access the configuration when calling
init_app.
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 :file:`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 hard-coded files based on that.
An interesting pattern is also to use classes and inheritance for configuration:
class Config(object):
TESTING = False
class ProductionConfig(Config):
DATABASE_URI = 'mysql://user@localhost/foo'
class DevelopmentConfig(Config):
DATABASE_URI = "sqlite:////tmp/foo.db"
class TestingConfig(Config):
DATABASE_URI = 'sqlite:///:memory:'
TESTING = True
To enable such a config you just have to call into :meth:`~flask.Config.from_object`:
app.config.from_object('configmodule.ProductionConfig')
Note that :meth:`~flask.Config.from_object` does not instantiate the class object. If you need to instantiate the class, such as to access a property, then you must do so before calling :meth:`~flask.Config.from_object`:
from configmodule import ProductionConfig
app.config.from_object(ProductionConfig())
# Alternatively, import via string:
from werkzeug.utils import import_string
cfg = import_string('configmodule.ProductionConfig')()
app.config.from_object(cfg)
Instantiating the configuration object allows you to use @property in
your configuration classes:
class Config(object):
"""Base config, uses staging database server."""
TESTING = False
DB_SERVER = '192.168.1.56'
@property
def DATABASE_URI(self): # Note: all caps
return f"mysql://user@{self.DB_SERVER}/foo"
class ProductionConfig(Config):
"""Uses production database server."""
DB_SERVER = '192.168.19.32'
class DevelopmentConfig(Config):
DB_SERVER = 'localhost'
class TestingConfig(Config):
DB_SERVER = 'localhost'
DATABASE_URI = 'sqlite:///:memory:'
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 to push code and configuration separately to the production server(s).
.. versionadded:: 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 :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 /instanceInstalled module or package:
$PREFIX/lib/pythonX.Y/site-packages/myapp $PREFIX/var/myapp-instance
$PREFIXis the prefix of your Python installation. This can be/usror the path to your virtualenv. You can print the value ofsys.prefixto 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 instance 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()