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

In this recipe, we'll take the application from the previous recipe, Composition of views and models, as a reference and modify it to work using blueprints.

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

First, we will start with the following 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, which should look as follows:

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 now defined a blueprint in the flask_app/my_app/hello/views.py file. We no longer need the application object anymore, and our complete routing is defined on a blueprint named hello. Instead of @app.route, we use @hello.route. The same blueprint is imported into flask_app/my_app/__init__.py and registered on the application object.

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

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

The previous recipe, Composition of views and models, is useful for further understanding how this recipe is useful.