Flask : Lightweight WSGI Web Application framework

Flask is a “micro” web framework, meaning it’s lightweight and easy to get started with, but it can scale to very complex applications. Using it essentially involves creating a Python script that “routes” specific web addresses (URLs) to Python functions.

1. Installation

First, ensure you have Flask installed via pip:

pip install flask

2. Create Your First App

Create a file named app.py. This is the skeleton of a basic Flask application:

from flask import Flask

# Initialize the Flask application
app = Flask(__name__)

# Define a "route" (the URL path)
@app.route("/")
def home():
    return "<h1>Hello, Flask!</h1>"

# Run the app if this file is executed directly
if __name__ == "__main__":
    app.run(debug=True)

3. Running the Server

In your terminal, run the script:

python app.py

You will see an output like Running on [http://127.0.0.1:5000](http://127.0.0.1:5000). Open that link in your browser, and you’ll see “Hello, Flask!”.

4. Key Concepts to Understand

Routing & Dynamic URLs

You can map different URLs to different functions. You can also capture parts of the URL as variables:

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

Rendering HTML (Templates)

Instead of returning raw strings, you usually want to return full HTML files. Flask uses the Jinja2 engine for this.

1. Create a folder named templates.
2. Inside it, create index.html.
3. Use render_template in your Python file:

from flask import render_template

@app.route("/")
def index():
    return render_template("index.html", title="Home Page")

The Development Server (debug=True)

When you use debug=True, two things happen:

• Auto-Reload: The server restarts itself every time you save a change to your code.
• Debugger: If your code crashes, it provides an interactive error page in the browser to help you find the bug.

Summary Checklist