Introduction to Flask

Flask is a lightweight and flexible Python web framework that is widely used for building web applications and APIs. It provides the tools needed to create simple yet powerful web apps.

In this lesson, you’ll learn the basics of Flask and how to set up a simple web application.

Installing Flask

To install Flask, use the following command in your terminal:

pip install flask

Once installed, you can import Flask in your Python scripts.

# Installing Flask
from flask import Flask

print("Flask imported successfully")

Creating a Flask Application

A Flask application starts with creating an instance of the Flask class:

# Creating a simple Flask app
from flask import Flask

app = Flask(__name__)

@app.route('/')
def home():
    return 'Welcome to Flask!'

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

Understanding Flask Routes

Routes in Flask define the URLs that trigger specific functions. Use the @app.route() decorator to define routes:

# Defining routes
@app.route('/')
def home():
    return 'Home Page'

@app.route('/about')
def about():
    return 'About Page'

Handling URL Parameters

Flask allows you to capture parts of the URL as parameters:

# Handling URL parameters
@app.route('/user/<username>')
def show_user(username):
    return f'User: {username}'

Returning HTML Responses

Flask allows you to return HTML responses directly:

# Returning HTML responses
@app.route('/hello')
def hello():
    return '<h1>Hello, Flask!</h1>'

Using Templates

Flask supports Jinja2 templates for rendering dynamic HTML content:

# Rendering a template
from flask import render_template

@app.route('/greet/<name>')
def greet(name):
    return render_template('greet.html', name=name)

Flask Debug Mode

Running the Flask app in debug mode provides better error messages and auto-reloads the app for code changes. Use debug=True in app.run().

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

Common Mistakes with Flask

Here are some mistakes to avoid:

• Not enabling debug mode during development.
• Hardcoding sensitive information like database credentials.
• Not using templates for dynamic content.

What Did We Learn?

In this lesson, you learned:

• How to install and set up Flask.
• How to define routes and handle URL parameters.
• How to return HTML responses and use templates.
• The importance of debug mode during development.

Great job! Let’s move to the next topic.