Flask is a web framework that allows web developers to build lightweight web applications quickly and easily with Flask Libraries. You can read more about it from these 2 weblinks:

• Flask Tutorial on Geeks for Geeks
• Official Flask website

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello_world():
    return "<p>Hello, World!</p>"

Let’s break down each line:

1. from flask import Flask: This line imports the Flask class from the Flask module. Flask is a web framework for Python that simplifies the process of building web applications.

2. app = Flask(__name__): This line creates an instance of the Flask class and assigns it to the variable app. The __name__ argument is a special variable in Python that represents the name of the current module. In this context, it is used to help Flask determine the root path for the application.

3. @app.route("/"): This is a decorator in Python, denoted by the @ symbol. Decorators are a way to modify or enhance the behavior of functions or methods. In this case, it decorates the hello_world function, telling Flask that this function should be called when the root URL (“/”) of the web application is accessed.

4. def hello_world():: This line defines a function named hello_world. This function will be called when someone accesses the root URL (“/”) of the web application.

5. return "<p>Hello, World!</p>": This line is the body of the hello_world function. It returns a simple HTML string (“<p>Hello, World!</p>”) as the response to the client’s request. In a real-world application, this could be a more complex HTML template or dynamically generated content.

In summary, this code sets up a basic Flask web application with a single route (“/”) that, when accessed, returns a simple “Hello, World!” message wrapped in a paragraph tag. To run this application, you would typically follow it with something like app.run() to start the development server and make the application accessible through a web browser.

Let me explain a little bit more on some of the terms used above:

1. Class:
   • In object-oriented programming (OOP), a class is a blueprint for creating objects. It defines a set of attributes and behaviors that objects created from the class will have.
   • In the given code, Flask is a class from the Flask module. It serves as a blueprint for creating Flask web applications. By creating an instance of this class, you can use its methods and properties to configure and run a web application.
2. Instance:
   • An instance is a specific occurrence of an object created from a class. It is a concrete realization of the attributes and behaviors defined by the class.
   • In the code, app is an instance of the Flask class. By creating an instance of the Flask class, you can customize and run your specific web application.
3. Current Module:
   • In Python, the term “module” refers to a file containing Python code. The __name__ variable is a special variable in Python that represents the name of the current module.
   • In the given code, __name__ is used as an argument when creating the Flask instance (app). It helps Flask determine the root path for the application.
4. Root Path of the Application:
   • The root path of a web application is the starting point for URL routing. It is the base URL that, when accessed, triggers the associated route or view.
   • In the code, app = Flask(__name__) sets up the Flask application, and the __name__ argument helps Flask determine the root path for the application.
5. Root URL:
   • The root URL is the base URL of a website or web application. In the context of a Flask application, it often corresponds to the forward slash (“/”).
   • In the code, @app.route("/") is a route decorator that associates the hello_world function with the root URL (“/”). When a user accesses the root URL, the associated function (hello_world) is called to handle the request.

These terms are fundamental concepts in programming and web development, and understanding them is crucial when working with frameworks like Flask.

If you are interested in studying Flask, I highly recommend this book:

The Flask Mega-Tutorial

by Miguel Grinberg Available on Amazon!