# A regular program: one user, runs once
name = input("Your name: ")
marks = [int(x) for x in input("Marks (space separated): ").split()]
print(f"{name}, your average is {sum(marks) / len(marks):.1f}")

# pip install flask   — run: flask --app app run
from flask import Flask, request, jsonify

app = Flask(__name__)

@app.post("/average")
def average():
    data = request.get_json()
    marks = data.get("marks", [])
    if not marks:
        return jsonify(error="marks required"), 400
    return jsonify(name=data.get("name"), average=round(sum(marks) / len(marks), 1))

# Same calculation — but now it waits for requests from many users

# Watch a real request and response
curl -i https://jsonplaceholder.typicode.com/posts/1
# HTTP/2 200
# content-type: application/json; charset=utf-8
# ...
# {"userId": 1, "id": 1, "title": "...", "body": "..."}

# Send data with POST
curl -i -X POST https://jsonplaceholder.typicode.com/posts \
  -H "Content-Type: application/json" -d '{"title":"hi","body":"test","userId":1}'

# Where web apps keep 'memory'
Cookie:   small data stored in the browser, sent with each request
Session:  server-side data linked to a cookie ID (e.g. logged-in user)
Database: permanent data (accounts, orders)
Token:    signed value (e.g. JWT) the client sends to prove identity

A regular program keeps state in variables; a web app must store it deliberately.

# pip install bcrypt
import bcrypt

password = b"S3cure!pass"
hashed = bcrypt.hashpw(password, bcrypt.gensalt())
print(hashed)                                  # store this, never the plain password
print(bcrypt.checkpw(b"S3cure!pass", hashed))  # True
print(bcrypt.checkpw(b"wrong", hashed))        # False

Before putting a web app online
☐ Debug mode off
☐ Secrets in environment variables
☐ HTTPS enabled
☐ Input validation on every endpoint
☐ Passwords hashed
☐ Error logging set up
☐ Database backups
☐ A way to roll back a bad release

Programmer → web developer plan
1. HTML and CSS basics (structure and layout)
2. HTTP: methods, status codes, headers, cookies
3. JavaScript in the browser: DOM and fetch
4. A backend framework in a language you know (Flask/Django, Express, Spring Boot)
5. Databases and SQL
6. Authentication and security basics
7. Deploy one app with a live URL

Regular programming → web equivalent
main() runs once          → server waits for requests
input()                   → form fields, request body
print()                   → HTML page or JSON response
variables hold state      → cookies, sessions, database
exceptions crash program  → return error status codes, keep server alive
files on disk             → database tables, object storage

First web project: Marks calculator
Frontend: HTML form for name and marks
Backend: POST /average returns JSON
Extras: validate input, show errors nicely
Stretch: save results in a database, add login
Deploy: free hosting tier with a live link