Education Blog: 🐍Building Web Applications with Flask: A Complete Beginner's Guide

Flask is a lightweight Python web framework that helps you build web applications quickly. Known for its simplicity and flexibility, Flask is perfect for beginners as well as experienced developers.

What is Flask?

Flask is called a micro web framework because it doesn’t include certain features like a database layer, form validation, or authentication by default. Instead, you can add what you need with external libraries. This makes Flask lightweight, easy to understand, and highly flexible.

Setting Up Flask

Before we start, ensure Python is installed on your system. Then, install Flask via pip:

pip install flask

Your First Flask Application

Create a file called app.py and add the following code:

from flask import Flask

app = Flask(__name__)

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

if __name__ == '__main__':
    app.run(debug=True, port=5000)

Save and run the file with python app.py.
Open your browser and go to http://127.0.0.1:5000/ to see "Hello, World!"

Understanding the Code

Importing Flask:

from flask import Flask

Imports the Flask class to create your app.

Creating an App Instance:

app = Flask(__name__)

__name__ helps Flask locate resources like templates.

Route Decorator:

@app.route('/')

Defines the URL path that triggers a function.

View Function:

def hello_world():
    return 'Hello, World!'

The function returns content to display in the browser.

Running the App:

app.run(debug=True, port=5000)

debug=True enables auto-reload and helpful error messages.
port=5000 specifies the port number.

Custom Host and Port

You can run Flask on a different host and port:

if __name__ == '__main__':
    app.run(host='0.0.0.0', port=8080, debug=True)

host='0.0.0.0' → makes it accessible on your network.
port=8080 → changes the default port from 5000.

Adding More Routes

You can define multiple routes:

from flask import Flask
app = Flask(__name__)

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

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

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

if __name__ == '__main__':
    app.run(debug=True, port=3000)

/user/<username> → captures a string variable from URL.
/post/<int:post_id> → captures an integer. Non-integer values return a 404.

Rendering HTML Templates

Flask uses Jinja2 to render HTML templates.

Create a folder called templates and a file index.html:

<!DOCTYPE html>
<html>
<head>
    <title>My Flask App</title>
</head>
<body>
    <h1>Hello, {{ name }}!</h1>
    <p>Welcome to my Flask application.</p>
</body>
</html>

Update your Flask code:

from flask import Flask, render_template

app = Flask(__name__)

@app.route('/')
def home():
    return render_template('index.html', name='Visitor')

@app.route('/user/<username>')
def show_user_profile(username):
    return render_template('index.html', name=username)

if __name__ == '__main__':
    app.run(debug=True, port=5001)

Pass variables from Flask to the template with {{ variable_name }}.
Flask automatically looks for HTML files in the templates folder.

Handling HTTP Methods

Flask can handle GET and POST requests:

from flask import Flask, request

app = Flask(__name__)

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        password = request.form['password']
        return f'Logged in as {username}'
    return '''
        <form method="post">
            <p><input type="text" name="username" placeholder="Username"></p>
            <p><input type="password" name="password" placeholder="Password"></p>
            <p><input type="submit" value="Login"></p>
        </form>
    '''

if __name__ == '__main__':
    app.run(debug=True, port=8000)

Redirects and URLs

from flask import Flask, redirect, url_for

app = Flask(__name__)

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

@app.route('/admin')
def admin():
    return 'Admin Page'

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

@app.route('/login/<name>')
def login(name):
    if name == 'admin':
        return redirect(url_for('admin'))
    return redirect(url_for('profile', username=name))

if __name__ == '__main__':
    app.run(debug=True, port=9000)

redirect() → redirects the user.
url_for() → generates URLs dynamically.

Handling Errors

Create custom error pages:

from flask import Flask, render_template

app = Flask(__name__)

@app.errorhandler(404)
def page_not_found(error):
    return render_template('404.html'), 404

@app.errorhandler(500)
def internal_error(error):
    return render_template('500.html'), 500

if __name__ == '__main__':
    app.run(debug=True, port=5002)

templates/404.html:

<!DOCTYPE html>
<html>
<head>
    <title>Page Not Found</title>
</head>
<body>
    <h1>Oops! Page not found.</h1>
    <p>The page you're looking for doesn't exist.</p>
</body>
</html>

Using Static Files

Project structure:

/myapp
    /templates
    /static
        /css
        /js
        /images
    app.py

Reference static files in templates:

<link rel="stylesheet" href="{{ url_for('static', filename='css/style.css') }}">
<script src="{{ url_for('static', filename='js/script.js') }}"></script>
<img src="{{ url_for('static', filename='images/logo.png') }}" alt="Logo">

Cookies and Sessions

from flask import Flask, request, make_response, session

app = Flask(__name__)
app.secret_key = 'your-secret-key'

@app.route('/set-cookie')
def set_cookie():
    resp = make_response('Cookie set!')
    resp.set_cookie('username', 'john-doe', max_age=60*60*24)
    return resp

@app.route('/login-session')
def login_session():
    session['username'] = 'john-doe'
    return 'Logged in!'

@app.route('/get-session')
def get_session():
    username = session.get('username', 'Guest')
    return f'Hello {username}'

if __name__ == '__main__':
    app.run(debug=True, port=5003)

Database Integration with Flask-SQLAlchemy

Install the extension:

pip install flask-sqlalchemy

Example usage:

from flask import Flask
from flask_sqlalchemy import SQLAlchemy

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

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(80), unique=True, nullable=False)
    email = db.Column(db.String(120), unique=True, nullable=False)

    def __repr__(self):
        return f'<User {self.username}>'

@app.route('/')
def index():
    users = User.query.all()
    return f'There are {len(users)} users in the database.'

if __name__ == '__main__':
    with app.app_context():
        db.create_all()
    app.run(debug=True, port=5004)

Running Flask via Command Line

export FLASK_APP=app.py   # Windows: set FLASK_APP=app.py
export FLASK_ENV=development
flask run --host=0.0.0.0 --port=8080

Conclusion

Flask is lightweight, simple, and flexible, making it perfect for building small to medium web applications.

Next steps for advanced learning:

Flask-WTF → Form handling
Flask-Login → Authentication
Flask-Mail → Sending emails
Flask-RESTful → APIs

🔹 9. Try it Yourself!

You can run the codes here and practice:

Happy coding with Flask! 🚀

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Saturday, August 23, 2025

🐍Building Web Applications with Flask: A Complete Beginner's Guide