flask/docs/config.rst

.. _config:

Configuration Handling
======================

.. versionadded:: 0.3

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:

.. tabularcolumns:: |p{6.5cm}|p{8.5cm}|

================================= =========================================
``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.
``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_KEY_ERRORS``   Werkzeug's internal data structures that
                                  deal with request specific data will
                                  raise special key errors that are also
                                  bad request exceptions.  By default
                                  these will be converted into 400
                                  responses which however can make
                                  debugging some issues harder.  If this
                                  config is set to ``True`` you will get
                                  a regular traceback instead.
================================= =========================================

.. admonition:: 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`_.

.. _bind: https://www.isc.org/software/bind

.. 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_KEY_ERRORS``, ``TRAP_HTTP_EXCEPTIONS``

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.

.. _fabric: http://fabfile.org/