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hg-review / bundled / flask / docs / patterns / appfactories.rst

Application Factories

If you are already using packages and modules for your application (:ref:`packages`) there are a couple of really nice ways to further improve the experience. A common pattern is creating the application object when the module is imported. But if you move the creation of this object, into a function, you can then create multiple instances of this and 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__)
    app.config.from_pyfile(config_filename)

    from yourapplication.views.admin import admin
    from yourapplication.views.frontend import frontend
    app.register_module(admin)
    app.register_module(frontend)

    return app

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

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

@admin.route('/')
def index():
    return render_template(current_app.config['INDEX_TEMPLATE'])

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

Using Applications

So to use such an application you then have to create the application first. Here an example run.py file that runs such an application:

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