Flask Web Applications & Templates

Hello Flask

Flask is a micro-framework because it doesn't require particular tools or libraries. It has no database abstraction layer, form validation, or any other components where pre-existing third-party libraries provide common functions.

from flask import Flask

app = Flask(__name__)

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

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

Routing

Routes are used to bind a function to a URL.

@app.route('/user/<username>')
def show_user_profile(username):
return f'User {username}'

Rendering Templates (Jinja2)

Flask uses the Jinja2 template engine to render HTML files.

from flask import render_template

@app.route('/hello/<name>')
def hello(name=None):
return render_template('hello.html', name=name)

Sample Template (hello.html)

<!doctype html>
<title>Hello from Flask</title>
{% if name %}
<h1>Hello {{ name }}!</h1>
{% else %}
<h1>Hello, World!</h1>
{% endif %}

Handling Forms

Access form data using the request object.

from flask import request

@app.route('/login', methods=['POST', 'GET'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        return f'Logging in {username}'
    return '''<form method="post">...</form>'''

✅ Practice (30 minutes)

Install Flask: pip install flask.
Create a simple app with 3 routes: Home, About, and Contact.
Use a shared base.html template with navigation and content blocks.
Create a route that accepts a dynamic id and fetches data (mocked) for that ID.