Python Docs

Flask

Flask is a lightweight Python web framework — simple to start, flexible to grow. You start with a small core and add only the components you need, which makes Flask ideal for APIs, microservices, and small to medium web applications.

Why Flask?

Minimal and easy to understand.
No strict project structure — you design it.
Choose your own ORM, auth, and extensions.
Perfect for REST APIs and microservices.
Great for learning how web frameworks work internally.

Install

Install Flask using pip:

Command

pip install flask

Hello, World

A minimal Flask application that responds with plain text at the root URL (/).

Example: Basic App

from flask import Flask
app = Flask(__name__)

@app.get('/')
def home():
    return 'Hello Flask'

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

How it works:

Flask(__name__) creates the app instance.
@app.get('/') maps the URL / to the home function.
debug=True enables hot reload and debugger (development only).

Templates (Jinja2)

Flask uses the Jinja2 templating engine to render dynamic HTML pages. You pass variables from the view to the template.

Example: Route with Template

# app.py
from flask import Flask, render_template
app = Flask(__name__)

@app.get('/hello/<name>')
def hello(name):
    return render_template('hello.html', name=name)

<!-- templates/hello.html -->
<!doctype html>
<h1>Hello {{ name }}!</h1>

Template flow:

Request: /hello/Alice
→ hello(name='Alice')
→ render_template('hello.html', name='Alice')
→ HTML: "Hello Alice!"

Notes & Best Practices

Use blueprints as your app grows to split routes into modules (e.g., auth, api, dashboard).
Set environment variables like FLASK_APP and FLASK_ENV for CLI usage:
```
export FLASK_APP=app.py
export FLASK_ENV=development
flask run
```
For production, disable debug and use a WSGI server like Gunicorn or uWSGI behind Nginx.
For APIs, consider using extensions like Flask-RESTful or Flask-Smorest.