Flask is a lightweight web framework for Python that allows developers to build web applications quickly and with minimal overhead. It is designed to be simple and flexible, making it an excellent choice for both beginners and experienced developers. Flask is often referred to as a "micro-framework" because it provides the essential components needed to build web applications without imposing a specific structure or requiring extensive boilerplate code.

1. Lightweight and Modular: Flask is designed to be lightweight and modular, allowing developers to choose the components they need and add extensions as required.
2. Built-in Development Server: Flask includes a built-in development server, making it easy to test and debug applications during development.
3. Jinja2 Templating: Flask uses the Jinja2 templating engine, which allows developers to create dynamic HTML pages with ease.
4. URL Routing: Flask provides a simple and intuitive way to map URLs to Python functions, making it easy to define the structure of your web application.
5. WSGI Compliance: Flask is WSGI (Web Server Gateway Interface) compliant, which means it can be deployed on any WSGI-compatible web server.
6. Extensible: Flask supports a wide range of extensions that add functionality such as database integration, form handling, authentication, and more.

• Simplicity: Flask's simplicity makes it an excellent choice for beginners who are just getting started with web development.
• Flexibility: Flask does not impose a specific project structure, allowing developers to organize their code in a way that makes sense for their application.
• Community and Ecosystem: Flask has a large and active community, with many extensions and resources available to help developers build robust applications.
• Performance: Flask's lightweight nature means it has a small footprint and can handle high-performance applications when needed.

Let's create a simple "Hello World" application in Flask to see how easy it is to get started.

1. Install Flask: First, you need to install Flask. You can do this using pip:

   pip install Flask
   

2. Create a Flask Application: Create a new Python file (e.g., app.py) and add the following code:

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

3. Run the Application: Open a terminal, navigate to the directory containing app.py, and run the following command:

   python app.py
   

4. Access the Application: Open a web browser and go to http://127.0.0.1:5000/. You should see the message "Hello, World!".

• Importing Flask: The from flask import Flask line imports the Flask class from the Flask module.
• Creating an Application Instance: app = Flask(__name__) creates an instance of the Flask class. The __name__ argument is used to determine the root path of the application.
• Defining a Route: The @app.route('/') decorator defines a route for the root URL (/). The hello_world function is called when this URL is accessed, and it returns the string "Hello, World!".
• Running the Application: The if __name__ == '__main__': block ensures that the application runs only if the script is executed directly (not imported as a module). The app.run(debug=True) line starts the development server with debug mode enabled.

In this section, we introduced Flask, a lightweight and flexible web framework for Python. We covered its key features, why you might choose Flask over other frameworks, and provided a practical example of creating a simple "Hello World" application. This sets the foundation for the rest of the course, where we will dive deeper into building more complex web applications with Flask.