Introduction to Flask

Build web apps and JSON APIs with Flask — routes, dynamic URLs, query params, and returning JSON responses in minutes.

"Flask is a micro web framework — micro meaning it starts small and you add only what you need. A working web server in 5 lines, a full REST API in 30."

— Shurai

How a Web Framework Works

Without a framework, you’d need to manually parse HTTP requests, build responses, and manage connections. Flask handles all of that — you just tell it which URL calls which function:

Request-response cycle with Flask:

Browser
GET /about

→

Flask Router
matches @app.route

→

Your Function
def about():

→

Response
HTML or JSON

terminal — install

pip install flask

Your First Flask App — 5 Lines

python — app.py

from flask import Flask

app = Flask(__name__)           # create the app

@app.route("/")                 # when someone visits "/"
def home():
    return "Hello, World!"       # return this response

@app.route("/about")
def about():
    return "<h1>About Us</h1><p>Built with Flask!</p>"

if __name__ == "__main__":
    app.run(debug=True)           # start the server

# Visit: http://127.0.0.1:5000
# Visit: http://127.0.0.1:5000/about

Dynamic URLs — Capture Values from the Path

Use <variable> in the route to capture part of the URL and pass it to your function:

python

@app.route("/user/<name>")
def profile(name):
    return f"<h1>Profile: {name}</h1>"
# /user/Riya  →  <h1>Profile: Riya</h1>

@app.route("/post/<int:post_id>")   # <int:...> converts to integer
def show_post(post_id):
    return f"Showing post #{post_id}"
# /post/42   →  Showing post #42
# /post/abc  →  404 (not an integer)

Reading Query Parameters and POST Data

python

from flask import Flask, request

# Query params: /search?q=python&lang=en
@app.route("/search")
def search():
    query = request.args.get("q", "")   # "" = default if missing
    lang  = request.args.get("lang", "en")
    return f"Searching '{query}' in {lang}"

Building a JSON REST API

This is Flask’s most common use in modern apps — a backend that speaks JSON. Use jsonify() to return proper JSON responses:

python — a complete mini books API

from flask import Flask, jsonify, request

app   = Flask(__name__)
books = [
    {"id": 1, "title": "The Alchemist",  "author": "Coelho"},
    {"id": 2, "title": "Atomic Habits",    "author": "Clear"},
    {"id": 3, "title": "Sapiens",           "author": "Harari"},
]

# GET /api/books  →  all books
@app.route("/api/books")
def get_all():
    return jsonify(books)

# GET /api/books/2  →  one book by id
@app.route("/api/books/<int:book_id>")
def get_one(book_id):
    book = next((b for b in books if b["id"] == book_id), None)
    if book is None:
        return jsonify({"error": "not found"}), 404
    return jsonify(book)

# POST /api/books  →  add a new book
@app.route("/api/books", methods=["POST"])
def add_book():
    data     = request.get_json()
    new_book = {"id": len(books) + 1, **data}
    books.append(new_book)
    return jsonify(new_book), 201   # 201 = Created

⚠️ debug=True is for development only

When debug=True, Flask auto-reloads on code changes and shows detailed error pages. Never use it in production — those detailed error pages expose your source code to anyone who triggers an error.

"The @app.route decorator is one of the most elegant uses of Python decorators in the real world. It maps a URL to a function — that simple idea is the backbone of most web frameworks."

— Shurai

🧠 Quiz — Q1

What does @app.route("/about") do?

🧠 Quiz — Q2

In the route @app.route("/post/<int:post_id>"), what does int: do?

🧠 Quiz — Q3

What does jsonify(data) do in Flask?

🧠 Quiz — Q4

A route needs to accept both GET and POST. How do you declare this?