New in version 0.9.
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 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:
On the contrast, during request handling, a couple of other rules exist:
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 current_app context local.
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 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.
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 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 url_for() function in case a SERVER_NAME was configured. This allows you to generate URLs even in the absence of a request.
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 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 Flask Extension Development.