Flask is a lightweight Python web framework that relies on Werkzeug for WSGI and Jinja2 for templating. It adopts a micro core design, allowing developers to add extensions for features like database integration or form validation as needed.

Environment Setup

A virtual environment is recommended for isolation:

python -m venv flask_env
source flask_env/bin/activate   # Linux / macOS
flask_env\Scripts\activate      # Windows

Install Flask inside the environment:

pip install flask

If the default PyPI mirror is slow, use a regional mirror with -i.

Minimal Application

Create a file named server.py:

from flask import Flask

service = Flask(__name__)

@service.route('/')
def home():
    return 'Hello from Flask!'

if __name__ == '__main__':
    service.run()

Run the script and access http://127.0.0.1:5000. The default port 5000 can be changed:

if __name__ == '__main__':
    service.run(host='0.0.0.0', port=8080)

Enable auto‑reloading during development to reflect code changes instantly:

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

The debug mode automatically restarts the server when files are modified, so you don't need to manually restart.

Routing Basics

Routes map URLs to view functions. The @app.route decorator defines these mappings. Dynamic segments can be captured with converters:

A more comprehensive example:

from flask import Flask, request

web_app = Flask(__name__)

@web_app.route('/greet')
def greet():
    return 'Welcome!'

@web_app.route('/items')
def show_items():
    return 'Item list'

# Integer converter
@web_app.route('/item/<int:item_id>/detail')
def item_detail(item_id):
    return str(item_id)

# String (default) converter
@web_app.route('/product/<product_name>/info')
def product_info(product_name):
    return product_name

# Float converter
@web_app.route('/price/above/<float:threshold>')
def price_filter(threshold):
    return str(threshold)

# Path converter
@web_app.route('/catalog/<path:category_path>')
def catalog(category_path):
    return category_path

# Allowing GET and POST
@web_app.route('/account', methods=['GET', 'POST'])
def account():
    if request.method == 'POST':
        return 'Creating account'
    return 'Account page'

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

alter the function names, URL patterns, and variable identifiers. The core ideas remain intact.

Testing the Routes

Start the server and visit these URLs:

1. http://127.0.0.1:5000/product/chair/info
2. http://127.0.0.1:5000/item/42/detail
3. http://127.0.0.1:5000/price/above/19.99
4. http://127.0.0.1:5000/catalog/electronics/laptops
5. http://127.0.0.1:5000/account (use GET in browser; test POST via tools like curl)

Method Restriction

By default, a route responds to GET requests only. To accept other methods, pass the methods parameter, for example methods=['GET', 'POST', 'PUT']. The view function can then inspect request.method to act accordingly.