Decorator Definition

A decorator in Python is a function that modifies the behavior of another function, without changing its code.

Think of a decorator as adding a layer of functionality before and/or after the original function runs.

Why Use Decorators?

Purpose	Example Use Case
Add functionality without editing original code	Logging, Authorization
Reuse common logic	Validation
Track performance	Execution time calculator
Restrict access	Admin/user roles

Basic Decorator Structure

def decorator_function(original_function):
    def wrapper_function():
        # Code before
        original_function()
        # Code after
    return wrapper_function

Applying a Decorator

@decorator_function
def display():
    print("Display function executed")

The @decorator_function applies extra behavior to the display() function.

Basic Example

def my_decorator(func):
    def wrapper():
        print("Before function runs")
        func()
        print("After function runs")
    return wrapper

@my_decorator
def greet():
    print("Hello!")

greet()

Output:

Before function runs
Hello!
After function runs

Decorator With Arguments

def decorator(func):
    def wrapper(*args, **kwargs):
        print("Arguments:", args, kwargs)
        return func(*args, **kwargs)
    return wrapper

@decorator
def add(a, b):
    print("Sum =", a + b)

add(5, 3)

Decorator Returning a Value

def decorator(func):
    def wrapper(*args):
        result = func(*args)
        print("Function returned:", result)
        return result
    return wrapper

@decorator
def multiply(x, y):
    return x * y

multiply(4, 5)

Real-World Example – Logging

def logger(func):
    def wrapper(*args, **kwargs):
        print(f"Running function: {func.__name__}")
        return func(*args, **kwargs)
    return wrapper

@logger
def show():
    print("Inside show function")

show()

Example – Execution Time Calculator

import time

def timer(func):
    def wrapper(*args, **kwargs):
        start = time.time()
        result = func(*args, **kwargs)
        end = time.time()
        print(f"Execution Time: {end-start:.5f} seconds")
        return result
    return wrapper

@timer
def slow_function():
    time.sleep(2)

slow_function()

Authentication / Access Control

def require_admin(func):
    def wrapper(user):
        if user != "admin":
            print("Access Denied!")
        else:
            return func(user)
    return wrapper

@require_admin
def delete_data(user):
    print("Data deleted successfully!")

delete_data("guest")
delete_data("admin")

Decorators with Parameters (Advanced)

Sometimes we need the decorator to accept its own argument:

def repeat(num):
    def decorator(func):
        def wrapper():
            for _ in range(num):
                func()
        return wrapper
    return decorator

@repeat(3)
def hello():
    print("Hello")

hello()

Output

Hello
Hello
Hello

Nesting Multiple Decorators

def bold(func):
    def wrapper():
        print("<b>", end="")
        func()
        print("</b>")
    return wrapper

def italic(func):
    def wrapper():
        print("<i>", end="")
        func()
        print("</i>")
    return wrapper

@bold
@italic
def text():
    print("Decorated Text", end="")

text()

Python Decorators