What Is Flask

Introduction

Flask is a Python web framework that stays small on purpose—you add only what your project needs. It handles HTTP routing, requests, responses, and hooks into Jinja2 templates, while leaving databases, auth, and admin panels to extensions you choose. This chapter explains what Flask is good for, how it compares to Django and FastAPI, and where this tutorial fits on Hello Code.

Prerequisites

Basic Python knowledge: functions, modules, imports
A terminal on Windows, macOS, or Linux
No Flask installed yet for this overview

What Flask Does

When a browser requests a URL, Flask:

text

  Browser          Flask app (Python)          Your code
     │                    │                        │
     │  GET /hello        │                        │
     ├───────────────────►│  match route           │
     │                    ├───────────────────────►│ return "Hello"
     │◄───────────────────┤ response               │

Code explanation:

Flask maps URLs to Python functions called view functions
The return value becomes the HTTP response body (HTML, JSON, or plain text)

Flask is a microframework: the core is tiny; you plug in Flask-SQLAlchemy, Flask-Login, and other packages as needed.

WSGI (Concept)

Production Python web apps speak WSGI—a standard interface between your Flask code and servers like Gunicorn:

text

  Gunicorn (HTTP)  →  WSGI  →  Flask application  →  your views

Code explanation:

Development uses flask run (Werkzeug server)—fine for learning, not for production
Deployment chapters use Gunicorn behind Nginx

Flask vs Django vs FastAPI

	Flask	Django	FastAPI
Style	Microframework, you assemble	Batteries included	Async API-first
Admin, ORM	Via extensions	Built-in	Via extensions
Templates	Jinja2 (built-in)	Django templates	Usually JSON only
Best for	APIs + small/medium sites	Large CMS, admin-heavy apps	High-concurrency async APIs
This track	Main focus	Mentioned only	Compare only

Many teams pick Flask when they want control and a flat learning curve before adding complexity.

Typical Use Cases

REST APIs for mobile or SPA frontends
Internal tools and dashboards
Prototypes and hackathon demos
Teaching HTTP and web architecture
Microservices alongside Java Spring Boot or Node backends

Flask 3 and Python Version

Use Python 3.10+ and Flask 3.x:

bash

python --version

Install paths are in the next chapter; see Python installation for your OS.

Hello Code Learning Path

text

Python basics → Flask (this track) → MySQL + SQLAlchemy
       ↓              ↓                    ↓
    pip/venv      routes/templates      data layer
       ↓              ↓                    ↓
     Git           auth/deploy          production DB

Track	Role
Python	Language syntax
MySQL	SQL and schema
Git	Version control
Linux	Server deploy
Nginx	Reverse proxy
Vue	SPA frontend consuming JSON APIs (CORS chapter pairs with Vue 17)

Minimal Preview

You will write code like this in chapter 3:

python

from flask import Flask
 
app = Flask(__name__)
 
@app.route("/")
def home():
    return "Hello, Flask!"
 
if __name__ == "__main__":
    app.run(debug=True)

Code explanation:

Flask(__name__) tells Flask where to find templates and static files
@app.route registers a URL path
The function return value is sent to the client

Tip

Start Simple, Grow Deliberately

Flask projects often begin as one file, then split into Blueprints and an application factory—this track follows that path.

FAQ

Flask still popular?

Yes—widely used for APIs, data tools, and education; Flask 3 keeps the project modern.

Need Django first?

No—pick Django if you want an all-in-one framework day one; Flask if you want to see each layer clearly.

Async Flask?

Flask 3 supports async views in some cases; this track focuses on sync WSGI patterns that match most tutorials and deployments.

Flask vs FastAPI for APIs?

FastAPI excels at typed async APIs; Flask excels at mixed HTML + JSON apps and huge extension ecosystem.

Commercial use?

Flask is BSD-licensed—free for commercial projects.

Where is official docs?

Pallets Flask documentation—authoritative reference alongside this track.