The Application Context
One of the design ideas behind Flask is that there are two different “states” in which code is executed. The application setup state in which the application implicitly is on the module level. It starts when the :class:`Flask` object is instantiated, and it implicitly ends when the first request comes in. While the application is in this state a few assumptions are true:
- the programmer can modify the application object safely.
- no request handling happened so far
- you have to have a reference to the application object in order to modify it, there is no magic proxy that can give you a reference to the application object you're currently creating or modifying.
In the contrast, during request handling, a couple of other rules exist:
- while a request is active, the context local objects (:data:`flask.request` and others) point to the current request.
- any code can get hold of these objects at any time.
There is a third state which is sitting in between a little bit. Sometimes you are dealing with an application in a way that is similar to how you interact with applications during request handling just that there is no request active. Consider for instance that you're sitting in an interactive Python shell and interacting with the application, or a command line application.
The application context is what powers the :data:`~flask.current_app` context local.
Purpose of the Application Context
The main reason for the application's context existence is that in the past a bunch of functionality was attached to the request context in lack of a better solution. Since one of the pillar's of Flask's design is that you can have more than one application in the same Python process.
So how does the code find the “right” application? In the past we recommended passing applications around explicitly, but that caused issues with libraries that were not designed with that in mind.
A common workaround for that problem was to use the :data:`~flask.current_app` proxy later on, which was bound to the current request's application reference. Since however creating such a request context is an unnecessarily expensive operation in case there is no request around, the application context was introduced.
Creating an Application Context
To make an application context there are two ways. The first one is the implicit one: whenever a request context is pushed, an application context will be created alongside if this is necessary. As a result of that, you can ignore the existence of the application context unless you need it.
The second way is the explicit way using the :meth:`~flask.Flask.app_context` method:
from flask import Flask, current_app app = Flask(__name__) with app.app_context(): # within this block, current_app points to app. print current_app.name
The application context is also used by the :func:`~flask.url_for` function in case a SERVER_NAME was configured. This allows you to generate URLs even in the absence of a request.
Locality of the Context
The application context is created and destroyed as necessary. It never moves between threads and it will not be shared between requests. As such it is the perfect place to store database connection information and other things. The internal stack object is called :data:`flask._app_ctx_stack`. Extensions are free to store additional information on the topmost level, assuming they pick a sufficiently unique name.
For more information about that, see :ref:`extension-dev`.
The context is typically used to cache resources on there that need to be created on a per-request or usage case. For instance database connects are destined to go there. When storing things on the application context unique names should be chosen as this is a place that is shared between Flask applications and extensions.
The most common usage is to split resource management into two parts:
- an implicit resource caching on the context.
- a context teardown based resource deallocation.
Generally there would be a get_X() function that creates resource X if it does not exist yet and otherwise returns the same resource, and a teardown_X() function that is registered as teardown handler.
This is an example that connects to a database:
import sqlite3 from flask import _app_ctx_stack def get_db(): top = _app_ctx_stack.top if not hasattr(top, 'database'): top.database = connect_to_database() return top.database @app.teardown_appcontext def teardown_db(exception): top = _app_ctx_stack.top if hasattr(top, 'database'): top.database.close()
The first time get_db() is called the connection will be established. To make this implicit a :class:`~werkzeug.local.LocalProxy` can be used:
from werkzeug.local import LocalProxy db = LocalProxy(get_db)
That way a user can directly access db which internally calls get_db().