Flask Introduction

What is Flask?

The Python micro framework for building web applications.

Flask is a Python framework for building web applications. It is lightweight and modular, and is a great tool for building simple websites. In this guide, we will build a simple website using Flask.

Knowledge Required

Basic familiarity with Python, HTML/CSS and SQL will be needed to build this website and resources can be found here:

Getting Started

Installing flask

To install Flask, you can use pip, which is the package manager for Python. Run the following command in your terminal:

pip install Flask

Basic Flask App

Flask applications are relatively simple to create. Here is a basic Flask app that displays “Hello, World!” on the webpage:

from flask import Flask

app = Flask(__name__)

@app.route("/")
def hello():
    return "Hello, World!"

To run the Flask app, save the code in a file called app.py and run the following command in your terminal:

$ flask run
 * Debug mode: off
WARNING: This is a development server. Do not use it in a production deployment. Use a production WSGI server instead.
 * Running on http://127.0.0.1:5000
Press CTRL+C to quit

This will start a development server on your local machine, and you can access the website by visiting http://127.0.0.1:5000 in your browser. You should see the text “Hello, World!” displayed on the webpage.

So how does this work?

Import the Flask class from the flask module.
```
from flask import Flask;
```
This imports the Flask class from the Flask module, which is used to create a Flask application.
Create the Flask app.
```
app = Flask(__name__)
```
This creates a new Flask application instance. The __name__ argument is a special Python variable that is set to the name of the current module. This is used by Flask to determine the root path of the application.
Define a route.
```
@app.route('/')
```
Flask uses things called decorators @app.route('/') to map functions to different webpages (routes). In this case, we are mapping the index function to the root URL /.
Define the index function.
```
def hello():
    return 'Hello, World!'
```
When a user visits the root URL, the hello function is called, and it returns the text Hello, World!. For more complex applications, you can return HTML or JSON data.