Blueprint routes and templates in Flask - Step-by-Step Execution

Concept Flow - Blueprint routes and templates

Create Blueprint

↓

Define routes in Blueprint

↓

Register Blueprint in app

↓

Run Flask app

↓

Request URL matching Blueprint route

↓

Render template from Blueprint folder

This flow shows how a Flask Blueprint is created, routes are defined inside it, then registered with the main app, and finally how a request triggers rendering a template from the Blueprint.

Execution Sample

Flask

from flask import Flask, Blueprint, render_template
app = Flask(__name__)
bp = Blueprint('bp', __name__, template_folder='templates')
@bp.route('/')
def home():
    return render_template('home.html')

app.register_blueprint(bp, url_prefix='/bp')

if __name__ == '__main__':
    app.run()

Defines a Blueprint with a route '/' that renders 'home.html', then registers it under '/bp' prefix in the main app.

Execution Table

Step	Action	Evaluation	Result
1	Create Blueprint 'bp'	Blueprint object created	bp ready with template_folder='templates'
2	Define route '/' in bp	Route '/' added to bp	bp has route '/'
3	Register bp with app at '/bp'	bp routes prefixed with '/bp'	app routes include '/bp/'
4	Run Flask app	App listens for requests	Server running
5	Request URL '/bp/'	Matches bp route '/' with prefix	Calls home() function
6	home() calls render_template('home.html')	Template searched in bp's templates folder	HTML content of home.html returned
7	Response sent to browser	Browser displays rendered page	User sees home page
8	Request URL '/bp/unknown'	No matching route	404 Not Found error

💡 Execution stops after response sent or 404 error if route not found

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 5	After Step 6	Final
bp	None	Blueprint object	Blueprint with route '/'	Registered with app	Route matched on request	Template rendered	Response sent
app	Flask app	Flask app	Flask app	Flask app with bp routes	Running server	Running server	Running server

Key Moments - 3 Insights

Why does the URL '/bp/' match the Blueprint route '/'?

Because the Blueprint is registered with url_prefix='/bp', all its routes get that prefix. So '/' inside Blueprint becomes '/bp/' in the app (see execution_table step 3 and 5).

Where does Flask look for the template 'home.html' when using Blueprint?

Flask looks inside the Blueprint's template_folder directory specified when creating the Blueprint (step 6). This keeps templates organized per Blueprint.

What happens if a URL does not match any Blueprint route?

Flask returns a 404 Not Found error because no route matches the request (step 8).

Visual Quiz - 3 Questions

Look at the execution table, what is the result after Step 3?

A Blueprint routes are registered with the app with prefix '/bp/'

B Blueprint is created but not registered

C App is running and serving requests

D Template is rendered

💡 Hint

Check the 'Result' column in Step 3 of the execution_table

At which step does the template 'home.html' get rendered?

A Step 4

B Step 6

C Step 5

D Step 7

💡 Hint

Look for 'render_template' call in the 'Action' column

If the Blueprint was registered without url_prefix, what would be the URL to access 'home.html'?

A /home.html

B /bp/

C /

D /bp/home.html

💡 Hint

Refer to the explanation in key_moments about url_prefix effect on routes

Concept Snapshot

Blueprints organize routes and templates in Flask.
Create Blueprint with template_folder.
Define routes inside Blueprint.
Register Blueprint with app and optional url_prefix.
Request URL uses prefix + route path.
Templates load from Blueprint folder.

Full Transcript

In Flask, Blueprints help organize routes and templates separately from the main app. First, you create a Blueprint object and specify a folder for its templates. Then, you define routes inside this Blueprint using decorators. Next, you register the Blueprint with the main Flask app, optionally adding a URL prefix. When the app runs and receives a request matching the Blueprint's route (including prefix), it calls the route function. This function can render a template located in the Blueprint's template folder. If no route matches, Flask returns a 404 error. This structure keeps code and templates modular and easier to manage.