The Application Context¶
The application context keeps track of the application-level data during
a request, CLI command, or other activity. Rather than passing the
application around to each function, the
g proxies are accessed instead.
This is similar to the The Request Context, which keeps track of request-level data during a request. A corresponding application context is pushed when a request context is pushed.
Purpose of the Context¶
Flask application object has attributes, such as
config, that are useful to access within views and
CLI commands. However, importing the
within the modules in your project is prone to circular import issues.
When using the app factory pattern or
writing reusable blueprints or
extensions there won’t be an
app instance to
import at all.
Flask solves this issue with the application context. Rather than
referring to an
app directly, you use the
proxy, which points to the application handling the current activity.
Flask automatically pushes an application context when handling a
request. View functions, error handlers, and other functions that run
during a request will have access to
Flask will also automatically push an app context when running CLI
commands registered with
Lifetime of the Context¶
The application context is created and destroyed as necessary. When a Flask application begins handling a request, it pushes an application context and a request context. When the request ends it pops the request context then the application context. Typically, an application context will have the same lifetime as a request.
See The Request Context for more information about how the contexts work and the full life cycle of a request.
Manually Push a Context¶
If you try to access
current_app, or anything that uses it,
outside an application context, you’ll get this error message:
RuntimeError: Working outside of application context. This typically means that you attempted to use functionality that needed to interface with the current application object in some way. To solve this, set up an application context with app.app_context().
If you see that error while configuring your application, such as when
initializing an extension, you can push a context manually since you
have direct access to the
app_context() in a
with block, and everything that runs in the block will have access
def create_app(): app = Flask(__name__) with app.app_context(): init_db() return app
If you see that error somewhere else in your code not related to configuring the application, it most likely indicates that you should move that code into a view function or CLI command.
The application context is a good place to store common data during a
request or CLI command. Flask provides the
g object for this
purpose. It is a simple namespace object that has the same lifetime as
an application context.
g name stands for « global », but that is referring to the
data being global within a context. The data on
g is lost
after the context ends, and it is not an appropriate place to store
data between requests. Use the
session or a database to
store data across requests.
A common use for
g is to manage resources during a request.
Xif it does not exist, caching it as
teardown_X()closes or otherwise deallocates the resource if it exists. It is registered as a
For example, you can manage a database connection using this pattern:
from flask import g def get_db(): if 'db' not in g: g.db = connect_to_database() return g.db @app.teardown_appcontext def teardown_db(exception): db = g.pop('db', None) if db is not None: db.close()
During a request, every call to
get_db() will return the same
connection, and it will be closed automatically at the end of the
You can use
LocalProxy to make a new context
from werkzeug.local import LocalProxy db = LocalProxy(get_db)
db will call
get_db internally, in the same way that
If you’re writing an extension,
g should be reserved for user
code. You may store internal data on the context itself, but be sure to
use a sufficiently unique name. The current context is accessed with
_app_ctx_stack.top. For more information see
Flask Extension Development.