Flask is one of the most popular micro-frameworks for building web applications in Python, and much of its power comes from the use of decorators. These decorators allow you to hook into the framework's functionality in a clean and readable way, without cluttering your code. In this post, we’ll explore the five most commonly used Flask decorators, plus two honorable mentions.

1 @app.route()

The @app.route() decorator is used to define the URL routing for your Flask app. It binds a URL to a Python function that handles the logic for that route.

Example:

@app.route('/')
def home():
return "Welcome to the homepage!"


This simple example maps the root URL (/) to the home() function, returning a welcome message to visitors.

2 @app.before_request()

This decorator runs a function before every request. It's perfect for checking authentication, initializing variables, or setting up resources.

Example:

@app.before_request
def check_authentication():
if not user_is_logged_in():
return "Please log in", 401


In this case, the function checks if a user is authenticated before allowing them to proceed.

3 @app.after_request()

The @app.after_request() decorator lets you modify the response after a request has been processed but before it's sent to the client. This is often used for tasks like adding headers or logging responses.

Example:

@app.after_request
def add_security_headers(response):
response.headers['X-Content-Type-Options'] = 'nosniff'
return response


Here, a security header is added to every response.

4 @app.errorhandler()

Error handling is crucial for any web app. The @app.errorhandler() decorator allows you to define custom behavior for specific HTTP status codes, like 404 Not Found.

Example:

@app.errorhandler(404)
def page_not_found(e):
return "Oops! Page not found.", 404


This custom handler returns a user-friendly message whenever a 404 error occurs.

5 @app.teardown_request()

Used for cleaning up after a request, @app.teardown_request() ensures resources like database connections are properly closed.

Example:

@app.teardown_request
def close_db_connection(exception=None):
db_session.remove()


Here, a database session is closed after each request, even if an exception was raised during request processing.

Honorable Mentions

While these decorators aren’t always used as frequently, they can be incredibly useful in certain cases:

@app.before_first_request()

This decorator is used to execute a function before the first request is handled. It’s ideal for initializing global resources, such as database connections or loading configuration.

@app.before_first_request
def initialize_db():
db.connect()
@app.context_processor()


If you need to inject variables into all your Jinja2 templates globally, use the @app.context_processor() decorator. This makes values like the current user or app configurations available in every template.

@app.context_processor
def inject_user():
return dict(current_user=get_current_user())


With this, current_user is available in all your templates without having to pass it manually every time.

Wrapping Up

Flask’s decorators make it easy to manage the lifecycle of a web request, customize behavior, and extend functionality. Understanding how and when to use these common decorators will help keep your Flask code clean, maintainable, and powerful. Did we miss any of your favorite decorators? Let us know!

Sources

• Flask API

• Flask Routing

• Flask Error Handling

• Flask Request Hooks

• Flask Context Processors

• Flask Before First Request