Python Flask Basics

Installing Flask

Flask is a third-party package, so install it with pip inside your project first.

pip install flask

A Minimal Flask App

Every Flask app starts with a Flask instance and one or more routes — functions connected to a URL using the @app.route decorator. When someone visits that URL, the matching function runs and its return value becomes the response.

from flask import Flask

app = Flask(__name__)

@app.route("/")
def home():
    return "Hello from Flask!"

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

python app.py

With debug=True, Flask auto-reloads whenever you save the file and shows helpful error pages, which is great while learning — just remember to switch it off before deploying to production.

Returning JSON

For an API endpoint, return a Python dictionary and Flask automatically converts it to a proper JSON response using jsonify.

from flask import Flask, jsonify

app = Flask(__name__)

@app.route("/api/status")
def status():
    return jsonify({"status": "ok", "service": "myinternships"})

Dynamic Routes

You can capture parts of the URL as variables by wrapping a name in angle brackets in the route, letting one function handle many different URLs.

@app.route("/user/<username>")
def show_user(username):
    return f"Profile page for {username}"

@app.route("/job/<int:job_id>")
def show_job(job_id):
    return jsonify({"job_id": job_id})