Flask

Flask is a lightweight web framework to build web applications quickly.

Setting up a Flask project

Install Flask with pip:

pip install Flask

Create a new file app.py and write the following code to set up a basic Flask application:

from flask import Flask

app = Flask(__name__)

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

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

Then run the app using:

python app.py

The application will run on http://127.0.0.1:5000/.

Routing and handling requests

Routes in Flask define the endpoints of the application.

You can use decorators to specify which function handles which URL.

Example:

@app.route('/submit', methods=['POST'])
def submit():
    data = request.form['field_name']
    return f'Form data: {data}'

Working with templates (Jinja2)

Flask uses Jinja2 as the templating engine to render HTML. Jinja2 allows you to dynamically generate HTML content from templates.

Create an HTML template index.html:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Hello {{ name }}</title>
</head>
<body>
    <h1>Hello, {{ name }}!</h1>
</body>
</html>

Render it from Flask:

from flask import render_template

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

Handling forms and requests data

Flask easily handles form data using request.form for POST requests.

Example:

from flask import request

@app.route('/submit', methods=['POST'])
def submit():
    username = request.form['username']
    return f'Username: {username}'

You can also handle query parameters in a URL with request.args:

@app.route('/search')
def search():
    query = request.args.get('q')
    return f'Search results for: {query}'

Using Flask extensions: Flask-SQLAlchemy

Flask extensions add extra functionality to Flask applications.

Flask-SQLAlchemy allows ORM-based database integration.

from flask_sqlalchemy import SQLAlchemy

app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///site.db'
db = SQLAlchemy(app)

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    username = db.Column(db.String(100), unique=True, nullable=False)

@app.route('/add_user', methods=['POST'])
def add_user():
    new_user = User(username=request.form['username'])
    db.session.add(new_user)
    db.session.commit()
    return f'User {new_user.username} added!'

Using Flask extensions: Flask-WTF

Flask-WTF allows to handle forms with validation.

from flask_wtf import FlaskForm
from wtforms import StringField
from wtforms.validators import DataRequired

class MyForm(FlaskForm):
    name = StringField('Name', validators=[DataRequired()])

@app.route('/form', methods=['GET', 'POST'])
def form():
    form = MyForm()
    if form.validate_on_submit():
        return f'Hello, {form.name.data}!'
    return render_template('form.html', form=form)

Error handling and logging in Flask

Flask allows to manage errors and log important application events.

To handle specific HTTP errors, use error handlers:

import logging

app.logger.setLevel(logging.DEBUG)

@app.errorhandler(404)
def not_found(error):
    app.logger.debug('Error 404')
    return 'This page does not exist!', 404

Deployment of Flask applications

Flask is not intended to be used directly in production. To deploy it to production you must use WSGI servers like Gunicorn or uWSGI.

Install Gunicorn

pip install gunicorn

Run it:

gunicorn app:app

Last modified: 17 March 2025