Extensions¶

What are extensions and what is their purpose? An extension is a small piece of code that should ease the integration of a third party library. Many such extensions have been already developed and one can find them in the Flask Extension Registry. Everyone should check first if there is a usable extension before developing new one. When developing an extension, follow the Flask Extension Development guideline that will help you get your extension running.

Plugging an Existing Extension¶

All extensions are located in a package called flask_something where “something” is the name of the extension you want to bridge. So for example if you plan to add support for an extension named login, you would name your extension’s package invenio.ext.login.

Alternatively, if the extension only reads configuration from current_app.config, you can just list in EXTENSIONS option as flask_something:Something, where Something has to accept an application object as first argument:

class Something(object):
    def __init__(self, app, optional=None):
        pass

So what do extensions actually look like? An extension has to ensure that it works with multiple Flask application instances at the same time.

Code structure¶

So let’s get started with creating such an extension bridge. This example bridge will provide very basic support for Flask Login.

First we’ll create the following folder structure:

invenio/ext/login/
    __init__.py
    legacy_user.py
    README

Here’s the contents of the most important files:

__init__.py contains setup_app() function:

from .legacy_user import UserInfo

def setup_app(app):
    """Setup login extension."""

    # Let's create login manager.
    _login_manager = LoginManager(app)
    _login_manager.login_view = app.config.get('CFG_LOGIN_VIEW',
                                               'webaccount.login')
    _login_manager.anonymous_user = UserInfo

    @_login_manager.user_loader
    def _load_user(uid):
        """
        Function should not raise an exception if uid is not valid
        or User was not found in database.
        """
        return UserInfo(int(uid))

    return app

legacy_user.py contains implementation of the UserMixin object:

from flask_login import UserMixin
from werkzeug.datastructures import CallbackDict, CombinedMultiDict

class UserInfo(CombinedMultiDict, UserMixin):

    """
    This provides legacy implementations for methods that Flask-Login
    and Invenio 1.x expect user objects to have.
    """

    def __init__(self, uid=None, force=False):
        ...