Flask Routing Fundamentals

Flask maps URLs to Python functions using @app.route() decorators. Create dynamic web pages with URL parameters and HTTP methods.

URL	Decorator	Method
`/`	`@app.route('/')`	GET (Home)
`/user/123`	`@app.route('/user/')`	GET (Dynamic)
`/login`	`@app.route('/login', methods=['POST'])`	POST (Form)
`/api/users`	`@app.route('/api/users', methods=['GET', 'POST'])`	API (REST)

Hello World Flask App

🚀 python app.py → http://localhost:5000

http://localhost:5000

🏠 Home Page

Welcome to Flask!

📊 Status: Running
🛠️ Framework: Flask 2.3+
🔥 Ready for production

from flask import Flask app = Flask(__name__) @app.route('/') def home(): return '''

Welcome to Flask! 🚀

Server running at localhost:5000

''' @app.route('/about') def about(): return '

About Flask

Lightweight Python web framework

' @app.route('/user/') def user_profile(user_id): return f'

User Profile: {user_id}

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

HTTP Methods (REST)

GET - Read Data

@app.route('/api/users', methods=['GET'])
def get_users():
    users = ['Alice', 'Bob', 'Charlie']
    return {'users': users, 'count': len(users)}
                        

POST - Create Data

from flask import request

@app.route('/api/users', methods=['POST'])
def create_user():
    data = request.json
    return {'message': 'User created', 'user': data}, 201
                        

PUT - Update Data

@app.route('/api/users/', methods=['PUT'])
def update_user(id):
    data = request.json
    return {'message': f'User {id} updated'}
                        

DELETE - Remove Data

@app.route('/api/users/', methods=['DELETE'])
def delete_user(id):
    return {'message': f'User {id} deleted'}, 200
                        

Jinja2 Templates

http://localhost:5000/users

👥 Users Page

Alice Smith alice@email.com

Bob Johnson bob@email.com

from flask import Flask, render_template

app = Flask(__name__)

@app.route('/users')
def users():
    user_data = [
        {'name': 'Alice Smith', 'email': 'alice@email.com'},
        {'name': 'Bob Johnson', 'email': 'bob@email.com'}
    ]
    return render_template('users.html', users=user_data)

# templates/users.html
"""


UsersUsers ({{ users|length }})
    {% for user in users %}
    
        {{ user.name }}
        {{ user.email }}
    

    {% endfor %}


"""
                

Forms & Validation

from flask import Flask, request, render_template, redirect, url_for, flash
from werkzeug.security import generate_password_hash

app = Flask(__name__)
app.secret_key = 'dev-key'

@app.route('/register', methods=['GET', 'POST'])
def register():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        
        if len(password) < 6:
            flash('Password must be 6+ characters')
            return render_template('register.html')
        
        # Save user (database)
        hashed = generate_password_hash(password)
        flash('Registration successful!')
        return redirect(url_for('login'))
    
    return render_template('register.html')
                

📝 register.html Template

{% with messages = get_flashed_messages() %} {% if messages %}

{% endif %} {% endwith %}

Database Integration

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

app = Flask(__name__)
app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///blog.db'
db = SQLAlchemy(app)

class Post(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    title = db.Column(db.String(100), nullable=False)
    content = db.Column(db.Text, nullable=False)
    date = db.Column(db.DateTime, default=datetime.utcnow)

@app.route('/posts')
def posts():
    all_posts = Post.query.all()
    return render_template('posts.html', posts=all_posts)

@app.route('/create', methods=['POST'])
def create_post():
    title = request.form['title']
    content = request.form['content']
    post = Post(title=title, content=content)
    db.session.add(post)
    db.session.commit()
    return redirect(url_for('posts'))
                

Production Features

Blueprints (Modular Apps)

# auth/blueprint.py
from flask import Blueprint

auth_bp = Blueprint('auth', __name__)

@auth_bp.route('/login')
def login():
    return 'Login page'

# app.py
app.register_blueprint(auth_bp, url_prefix='/auth')
# Now: /auth/login
                

Environment Config

import os
from dotenv import load_dotenv

load_dotenv()

app.config['SECRET_KEY'] = os.getenv('SECRET_KEY')
app.config['DATABASE_URL'] = os.getenv('DATABASE_URL')

class Config:
    DEBUG = os.getenv('FLASK_DEBUG') == 'True'
    SQLALCHEMY_TRACK_MODIFICATIONS = False
                

Deployment

# requirements.txt
"""
Flask==2.3.3
Flask-SQLAlchemy==3.0.5
python-dotenv==1.0.0
Werkzeug==2.3.7
"""

# Procfile (Heroku)
web: gunicorn app:app

# runtime.txt
python-3.11.5

# wsgi.py (production)
from app import app
if __name__ == "__main__":
    app.run()
                

Heroku
Free tier available

Vercel
Serverless

Docker
Production ready

Production Projects

1. Blog Platform

CRUD posts, user auth, comments, search, pagination

2. Task Manager API

REST API + database + authentication + frontend

3. URL Shortener

Custom short URLs, analytics, QR codes, admin dashboard

4. E-commerce Store

Products, cart, checkout, payments, admin panel

Full-Stack Web Developer!

🌐 Production Web Applications:

📱 SaaS Dashboards
Analytics, users, billing

🛒 E-commerce Sites
Products, cart, payments

📊 REST APIs
Mobile + frontend integration

⚙️ Admin Panels
CRUD, reports, management

💼 Production Deployments
Heroku, Docker, AWS

Deploy full-stack web applications worldwide! 🚀

Flask Web Development