Application Factories

If you are already using packages and blueprints for your application (Modular Applications with Blueprints) there are a couple of really nice ways to further improve the experience. A common pattern is creating the application object when the blueprint is imported. But if you move the creation of this object, into a function, you can then create multiple instances of this app later.

So why would you want to do this?

  1. Testing. You can have instances of the application with different settings to test every case.
  2. Multiple instances. Imagine you want to run different versions of the same application. Of course you could have multiple instances with different configs set up in your webserver, but if you use factories, you can have multiple instances of the same application running in the same application process which can be handy.

So how would you then actually implement that?

Basic Factories

The idea is to set up the application in a function. Like this:

def create_app(config_filename):
    app = Flask(__name__)

    from yourapplication.model import db

    from yourapplication.views.admin import admin
    from yourapplication.views.frontend import frontend

    return app

The downside is that you cannot use the application object in the blueprints at import time. You can however use it from within a request. How do you get access to the application with the config? Use current_app:

from flask import current_app, Blueprint, render_template
admin = Blueprint('admin', __name__, url_prefix='/admin')

def index():
    return render_template(current_app.config['INDEX_TEMPLATE'])

Here we look up the name of a template in the config.

Factories & Extensions

It’s preferable to create your extensions and app factories so that the extension object does not initially get bound to the application.

Using Flask-SQLAlchemy, as an example, you should not do something along those lines:

def create_app(config_filename):
    app = Flask(__name__)

    db = SQLAlchemy(app)

But, rather, in (or equivalent):

db = SQLAlchemy()

and in your (or equivalent):

def create_app(config_filename):
    app = Flask(__name__)

    from yourapplication.model import db

Using this design pattern, no application-specific state is stored on the extension object, so one extension object can be used for multiple apps. For more information about the design of extensions refer to Flask Extension Development.

Using Applications

So to use such an application you then have to create the application first in a separate file otherwise the flask command won’t be able to find it. Here an example file that creates such an application:

from yourapplication import create_app
app = create_app('/path/to/config.cfg')

It can then be used with the flask command:

export FLASK_APP=exampleapp
flask run

Factory Improvements

The factory function from above is not very clever so far, you can improve it. The following changes are straightforward and possible:

  1. make it possible to pass in configuration values for unittests so that you don’t have to create config files on the filesystem
  2. call a function from a blueprint when the application is setting up so that you have a place to modify attributes of the application (like hooking in before / after request handlers etc.)
  3. Add in WSGI middlewares when the application is creating if necessary.