Python Flask Basics

Introduction

Flask is a lightweight Python web framework. It’s perfect for building APIs, dashboards, small web apps, bots backend endpoints, or full CodeTweakrs tools. Simple, flexible, and fast.

1. Installing Flask

pip install flask

2. Basic Flask App

from flask import Flask

app = Flask(__name__)

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

app.run()

Running the file starts a local server on http://127.0.0.1:5000.

3. Debug Mode (Auto Restart)

app.run(debug=True)

4. Returning HTML

@app.route("/dashboard")
def dash():
    return "
Dashboard Loaded
"

5. Returning JSON

from flask import jsonify

@app.route("/api/user")
def user():
    return jsonify({"name": "Kaloyan", "rank": "Pro"})

6. URL Parameters

@app.route("/hello/<name>")
def hello(name):
    return f"Hello {name}!"

7. Query Arguments

@app.route("/sum")
def summ():
    from flask import request
    a = int(request.args.get("a", 0))
    b = int(request.args.get("b", 0))
    return str(a + b)

8. Handling POST Requests

@app.route("/login", methods=["POST"])
def login():
    from flask import request
    data = request.json
    return {"status": "ok", "user": data.get("name")}

9. Templates (HTML Files)

Inside /templates/home.html:

<h1>Welcome, {{ user }}</h1>

Use it in Flask:

from flask import render_template

@app.route("/profile")
def profile():
    return render_template("home.html", user="Kaloyan")

10. Static Files (CSS/JS)

Create a /static folder.

<link rel="stylesheet" href="/static/style.css">

11. Structuring a Real Flask App

/project
  /static
  /templates
  app.py

12. Redirects

from flask import redirect

@app.route("/old")
def old():
    return redirect("/new")

13. Sessions (Login Cookies)

from flask import session

app.secret_key = "secret123"
session["user"] = "Kaloyan"

14. Error Handlers

@app.errorhandler(404)
def nf(e):
    return "Page not found", 404

15. Deploying Flask (Gunicorn + Nginx)

pip install gunicorn
gunicorn app:app

Use Nginx as reverse proxy in production.

Summary