Using Flask Extensions

Flask extensions can be used with Quart, with some caveats. To do so the very first import in your code must be import quart.flask_patch as this will add modules purporting to be Flask modules for later use by the extension. For example,

import quart.flask_patch

from quart import Quart
import flask_login

app = Quart(__name__)
login_manager = flask_login.LoginManager()
login_manager.init_app(app)

...

Caveats

Flask extensions must use the global request proxy variable to access the request, any other access e.g. via _get_current_object() will require asynchronous access. To enable this the request body must be fully received before any part of the request is handled, which is a limitation not present in vanilla flask.

Trying to use Flask alongside Quart in the same runtime will likely not work, and lead to surprising errors.

The flask extension must be limited to creating routes, using the request and rendering templates. Any other more advanced functionality may not work.

Synchronous functions will not run in a separate thread (unlike Quart normally) and hence may block the event loop.

Finally the flask_patching system also relies on patching asyncio, and hence other implementations or event loop policies are unlikely to work.

Supported extensions

A list of officially supported flask extensions exist here of those a few have been tested against Quart (the extensions tested are still supported and don’t require external services). The following flask extensions are tested and known to work with quart,

• Flask-BCrypt

• Flask-Caching

• Flask-KVSession

• Flask-Limiter

• Flask-Login See also Quart-Login

• Flask-Mail

• Flask-Mako

• Flask-Seasurf

• Flask-SQLAlchemy

• Flask-WTF

Broken extensions

The following flask extensions have been tested are known not to work with quart,

• Flask-CORS, as it uses app.make_response which must be awaited. Try Quart-CORS instead.

• Flask-Restful as it subclasses the Quart (app) class with synchronous methods overriding asynchronous methods. Try Quart-OpenApi or Quart-Schema instead.