Overview

Introduction to Flask

Flask is a lightweight Python web framework used to build web applications quickly and efficiently. It is designed to be simple, flexible, and easy to scale, making it one of the most popular frameworks for beginners and professionals.

Flask is based on:

• Python language

• WSGI (Web Server Gateway Interface)

• Jinja2 templating engine

It allows developers to build:

• Websites

• REST APIs

• Web dashboards

• Backend services

Why Use Flask?

Flask is widely used because it is:

• Simple to learn

• Minimal and flexible

• Highly customizable

• Suitable for small and large projects

It does not force a strict structure, so developers can design applications freely.

Installing Flask

To install Flask, use pip:

pip install flask

To verify installation:

python -m flask --version

Creating a Simple Flask App

A basic Flask application looks like this:

from flask import Flask

app = Flask(__name__)

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

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

What this does:

• Creates a Flask app

• Defines a route /

• Returns a response in the browser

• Runs a local development server

Flask Routes

Routes define URLs in your application.

Example:

@app.route('/about')
def about():
    return "About Page"

Now visiting:

http://127.0.0.1:5000/about

will show the About Page.

Dynamic Routes

Flask allows dynamic URLs:

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

Example:

/user/Ali → Hello, Ali!

HTTP Methods in Flask

Flask supports different HTTP methods:

• GET (default)

• POST

• PUT

• DELETE

Example:

from flask import request

@app.route('/submit', methods=['POST'])
def submit():
    data = request.form['name']
    return f"Received {data}"

Templates in Flask (HTML Rendering)

Flask uses Jinja2 templates to render HTML.

Folder structure:

/templates
    index.html

Flask code:

from flask import render_template

@app.route('/')
def home():
    return render_template('index.html')

HTML file:

<h1>Welcome to Flask</h1>

Passing Data to Templates

@app.route('/user/<name>')
def user(name):
    return render_template('user.html', username=name)

In HTML:

<h1>Hello {{ username }}</h1>

Static Files (CSS, JS, Images)

Flask uses a static folder:

/static
    style.css

Example:

<link rel="stylesheet" href="{{ url_for('static', filename='style.css') }}">

Forms in Flask

Flask handles form input using request.

Example:

from flask import request

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        username = request.form['username']
        return f"Welcome {username}"
    return render_template('login.html')

Flask with Databases

Flask can connect to databases like:

• SQLite

• MySQL

• PostgreSQL

Using SQLAlchemy:

pip install flask-sqlalchemy

Example Database Setup

from flask_sqlalchemy import SQLAlchemy

app.config['SQLALCHEMY_DATABASE_URI'] = 'sqlite:///data.db'
db = SQLAlchemy(app)

Creating Models

class User(db.Model):
    id = db.Column(db.Integer, primary_key=True)
    name = db.Column(db.String(100))

REST API with Flask

Flask is commonly used for APIs.

Example:

@app.route('/api')
def api():
    return {"message": "Hello API"}

JSON Response

from flask import jsonify

@app.route('/data')
def data():
    return jsonify({"name": "Flask", "type": "framework"})

Flask Project Structure

A basic structure:

project/
│
├── app.py
├── templates/
├── static/
├── models/
└── requirements.txt

Flask Debug Mode

Debug mode helps during development:

app.run(debug=True)

It allows:

• Auto-reload

• Error tracking

Deploying Flask Applications

Flask apps can be deployed using:

• Heroku

• AWS

• PythonAnywhere

• Docker

Advantages of Flask

• Lightweight

• Easy to learn

• Flexible structure

• Large community

• Great for APIs and small apps

Limitations of Flask

• Not ideal for very large projects without structure

• Requires manual setup for many features

• Less built-in functionality than Django

Best Use Cases

Flask is best for:

• API development

• Small to medium web apps

• Prototyping

• Microservices

Conclusion

Flask is a simple and powerful Python web framework that allows developers to build web applications and APIs quickly. Its flexibility, simplicity, and minimal structure make it an excellent choice for beginners and professionals who want full control over their projects.