Creating a modular web app with blueprints

A blueprint is a concept in Flask that helps make large applications really modular. They keep application dispatching simple by providing a central place to register all the components in the application. A blueprint looks like an application object but is not an application. It looks like a pluggable application or a smaller part of a bigger application, but it is not so. A blueprint is actually a set of operations that can be registered on an application and represents how to construct or build an application.

Getting ready

We will take the application from the previous recipe, Composition of views and models, as a reference and modify it to work using blueprints.

How to do it…

The following is an example of a simple Hello World application using blueprints. It will work in a manner similar to the previous recipe but is much more modular and extensible.

First, we will start with the flask_app/my_app/__init__.py file:

from flask import Flask
from my_app.hello.views import hello

app = Flask(__name__)
app.register_blueprint(hello)

Next, the views file, my_app/hello/views.py, will look like the following lines of code:

from flask import Blueprint
from my_app.hello.models import MESSAGES

hello = Blueprint('hello', __name__)


@hello.route('/')
@hello.route('/hello')
def hello_world():
    return MESSAGES['default']


@hello.route('/show/<key>')
def get_message(key):
    return MESSAGES.get(key) or "%s not found!" % key


@hello.route('/add/<key>/<message>')
def add_or_update_message(key, message):
    MESSAGES[key] = message
    return "%s Added/Updated" % key

We have defined a blueprint in the flask_app/my_app/hello/views.py file. We don't need the application object anymore here, and our complete routing is defined on a blueprint named hello. Instead of @app.route, we used @hello.route. The same blueprint is imported in flask_app/my_app/__init__.py and registered on the application object.

We can create any number of blueprints in our application and do most of the activities that we would do with our application, such as providing different template paths or different static paths. We can even have different URL prefixes or subdomains for our blueprints.

How it works…

This application will work in exactly the same way as the last application. The only difference is in the way the code is organized.

See also

• The previous recipe, Composition of views and models, is useful to get a background on how this recipe is useful.