Decorators in Python

Learn how Python decorators can add extra functionality to an existing function without changing its original code.

What is a Decorator?

A decorator is a function that adds extra behavior to another function.

Decorators allow us to modify or extend the behavior of a function without changing the original function code.

Decorator Flow

Basic Syntax

def decorator(function):

    def wrapper():

        function()

    return wrapper


@decorator
def welcome():

    print("Welcome to Python")

The @decorator syntax tells Python to apply the decorator to the function.

Simple Decorator Example

def message_decorator(function):

    def wrapper():

        print("Before function")

        function()

        print("After function")

    return wrapper


@message_decorator
def welcome():

    print("Welcome to Python")


welcome()

Output:

Before function
Welcome to Python
After function

The decorator adds statements before and after the original function.

Applying Decorator Without @

A decorator can also be applied directly by assigning the decorated function.

def decorator(function):

    def wrapper():

        print("Before")

        function()

        print("After")

    return wrapper


def welcome():

    print("Welcome")


welcome = decorator(welcome)

welcome()

Output:

Before
Welcome
After

Decorator with Function Parameters

A decorator can also work with functions that receive parameters.

def decorator(function):

    def wrapper(name):

        print("Welcome message")

        function(name)

    return wrapper


@decorator
def greet(name):

    print("Hello", name)


greet("Rahul")

Output:

Welcome message
Hello Rahul

Decorator with Multiple Arguments

We can use *args and **kwargs when the decorated function may receive different numbers of arguments.

def decorator(function):

    def wrapper(*args, **kwargs):

        print("Function started")

        result = function(*args, **kwargs)

        print("Function completed")

        return result

    return wrapper


@decorator
def add(a, b):

    return a + b


print(add(10, 20))

Output:

Function started
Function completed
30

Decorator with Return Value

A decorator can return the result produced by the original function.

def decorator(function):

    def wrapper(a, b):

        result = function(a, b)

        return result

    return wrapper


@decorator
def multiply(a, b):

    return a * b


print(multiply(5, 4))

Output:

20

Practical Example: Login Check

Decorators can be used to perform a check before allowing a function to execute.

def login_required(function):

    def wrapper(user):

        if user == "admin":

            return function(user)

        else:

            print("Access denied")

    return wrapper


@login_required
def dashboard(user):

    print("Welcome to dashboard")


dashboard("admin")

Output:

Welcome to dashboard

Multiple Decorators

More than one decorator can be applied to the same function.

def first(function):

    def wrapper():

        print("First decorator")

        function()

    return wrapper


def second(function):

    def wrapper():

        print("Second decorator")

        function()

    return wrapper


@first
@second
def welcome():

    print("Welcome")


welcome()

Output:

First decorator
Second decorator
Welcome

Preserving Function Information

The functools.wraps function can be used when we want to preserve information about the original function.

from functools import wraps

def decorator(function):

    @wraps(function)
    def wrapper():

        return function()

    return wrapper


@decorator
def welcome():

    print("Welcome to Python")


welcome()

This is commonly used when creating reusable decorators.

Common Uses of Decorators

Advantages of Decorators

Best Practices

Common Mistakes

• Forgetting to return the wrapper function.
• Forgetting to return the result of the original function.
• Not handling function arguments correctly.
• Creating very complicated decorators for simple tasks.
• Forgetting to use functools.wraps when function metadata needs to be preserved.

Practice Programs

1. Create a decorator that prints a message before a function runs.
2. Create a decorator that prints a message after a function runs.
3. Create a decorator that accepts function arguments.
4. Create a decorator for checking user login.
5. Create a decorator that calculates the execution time of a function.