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Module 28 - Flask Basics

Flask is a lightweight micro web framework for Python that makes it easy to build web applications quickly.

1. Introduction to Flask​

Installation​

pip install Flask

Why Flask?​

✅ Lightweight and flexible
✅ Easy to learn
✅ Great for small to medium projects
✅ RESTful request dispatching
✅ Built-in development server

2. Hello World​

from flask import Flask

app = Flask(__name__)

@app.route('/')
def hello():
    return 'Hello, World!'

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

Run: python app.py → Visit http://127.0.0.1:5000/

3. Routing​

from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return 'Home Page'

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

# Dynamic routes
@app.route('/user/<username>')
def user_profile(username):
    return f'Profile: {username}'

# Type constraints
@app.route('/post/<int:post_id>')
def show_post(post_id):
    return f'Post ID: {post_id}'

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

4. Templates​

templates/index.html:

<!DOCTYPE html>
<html>
<head>
    <title>{{ title }}</title>
</head>
<body>
    <h1>Welcome, {{ name }}!</h1>
    <ul>
    {% for item in items %}
        <li>{{ item }}</li>
    {% endfor %}
    </ul>
</body>
</html>

app.py:

from flask import Flask, render_template

app = Flask(__name__)

@app.route('/')
def home():
    return render_template('index.html', 
                         title='Home', 
                         name='Alice',
                         items=['Python', 'Flask', 'Web Dev'])

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

5. Forms and User Input​

from flask import Flask, render_template, request, redirect, url_for

app = Flask(__name__)

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        
        if username == 'admin' and password == 'secret':
            return redirect(url_for('dashboard'))
        else:
            return 'Invalid credentials'
    
    return render_template('login.html')

@app.route('/dashboard')
def dashboard():
    return 'Welcome to Dashboard!'

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

Summary​

✅ Flask is a micro web framework
✅ Routes map URLs to Python functions
✅ Jinja2 templates for HTML rendering
✅ Built-in development server
✅ Easy form handling

Next Steps​

In Module 29, you'll learn:

  • FastAPI framework
  • Building REST APIs
  • Automatic API documentation
  • Pydantic models

Practice Exercises​

  1. Create a simple blog with home, about, and contact pages
  2. Build a todo list app with add/delete functionality
  3. Implement user registration and login system
  4. Create a portfolio website with Flask
  5. Build a simple REST API for a todo app :::