Flask Tutorial

What is Flask?

Flask is a micro web framework written in Python. It’s designed to be simple and easy to use, making it an excellent choice for beginners and professionals alike. Unlike full-stack frameworks, Flask provides the essentials and leaves the rest to the developer's choice, offering great flexibility.

Why Use Flask?

Flask is lightweight, modular, and ideal for developing small to medium-sized applications. It gives developers full control over the components they want to use, allowing for more customized development. Flask also comes with a built-in development server, debugger, and supports secure cookies and RESTful request handling.

How to Install Flask

To get started with Flask, it's recommended to create a virtual environment. Here’s how you can set it up:

pip install virtualenv
virtualenv venv
# On Windows
venv\Scripts\activate
# On macOS/Linux
source venv/bin/activate
pip install Flask

This ensures that your project dependencies are kept separate from other Python projects on your machine.

Creating a Basic Flask Application

A basic Flask app can be created with just a few lines of code. Here's an example:

from flask import Flask

app = Flask(__name__)

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

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

This code starts a web server and responds with "Hello, Flask!" when accessed at the root URL.

Routing in Flask

Routing determines which function runs when a URL is visited. You can also add dynamic parts to the URL:

@app.route('/user/<username>')
def show_user_profile(username):
    return f'User: {username}'

This allows you to capture values directly from the URL and use them within your view functions.

Handling HTTP Methods

Flask allows you to handle different HTTP methods like GET and POST in your routes:

from flask import request

@app.route('/login', methods=['GET', 'POST'])
def login():
    if request.method == 'POST':
        return 'Logged in successfully!'
    return 'Login Page'

This makes it easy to handle form submissions and process user input.

Using Templates with Jinja2

Flask uses the Jinja2 template engine to dynamically generate HTML. Templates are stored in a templates folder:

from flask import render_template

@app.route('/about')
def about():
    return render_template('about.html', title='About Us')

This keeps your application organized by separating the business logic from the presentation layer.

Working with Forms

HTML forms can send data to Flask routes using POST or GET methods. Here’s an example form:

<form method="post">
  <input type="text" name="username">
  <input type="submit">
</form>

The submitted data can be accessed using request.form in your Flask route.

Running the Flask Application

To run the Flask app, save your Python script and run it in the terminal. Flask will start a local development server at http://127.0.0.1:5000. Visit this URL in your browser to see your app in action.

Conclusion

Flask is a powerful yet simple framework that allows you to build robust web applications quickly. Its flexibility and minimal design make it a preferred choice for many Python developers. Whether you’re building a basic website or a complex web application, Flask has the tools you need to get started.