Decorators in Python (Advanced)

A decorator is a function that modifies another function without changing its actual code.

Decorators are built using: - Functions - Closures - Higher-order functions

---

Basic Idea of Decorators

A decorator: - Takes a function as input - Adds extra behavior - Returns a new function

---

1. Function Decorators

Basic decorator example:

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


def say_hello():
    print("Hello")

decorated = decorator(say_hello)
decorated()

Output:

Before function runs
Hello
After function runs

---

Using @ syntax (Pythonic way)

def decorator(func):
    def wrapper():
        print("Before")
        func()
        print("After")
    return wrapper


@decorator
def greet():
    print("Good Morning")

greet()

---

Real World Example

Scenario	Decorator Use
Login check	authentication
Logging	request logs
Timing	performance check

---

Graph (Flow)

greet()
  |
  v
wrapper()
  |
  +--> Before logic
  +--> Original function
  +--> After logic

---

Common Mistake

@decorator()
def func():
    print("Hi")

Error: - Missing function reference

Correct:

@decorator

---

Homework 1

Create a decorator that prints: - "Start" - "End"

Solution:

def decorator(func):
    def wrapper():
        print("Start")
        func()
        print("End")
    return wrapper


@decorator
def hello():
    print("Hello World")

hello()

---

2. Decorator Chaining

Multiple decorators can be applied to one function.

Example:

def deco1(func):
    def wrapper():
        print("Decorator 1")
        func()
    return wrapper


def deco2(func):
    def wrapper():
        print("Decorator 2")
        func()
    return wrapper


@deco1
@deco2
def show():
    print("Original Function")

show()

Output:

Decorator 1
Decorator 2
Original Function

---

Execution Flow

@deco1
   ↓
@deco2
   ↓
function()

---

Real World Example

Layer	Use
Authentication	login check
Authorization	permission check
Logging	request logs

---

Homework 2

Create two decorators: - One prints "A" - Another prints "B"

Apply both to a function.

---

3. Decorators with Arguments

Sometimes decorators need parameters.

Example:

def repeat(n):
    def decorator(func):
        def wrapper():
            for i in range(n):
                func()
        return wrapper
    return decorator


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

hello()

Output:

Hello
Hello
Hello

---

Real World Example

Use Case	Example
Retry system	retry 3 times
Logging level	debug/info
Rate limiting	5 requests

---

Graph

repeat(n)
   ↓
decorator(func)
   ↓
wrapper()
   ↓
loop execution

---

Common Mistake

@repeat
def func():
    print("Hi")

Problem: - Missing argument

Correct:

@repeat(3)

---

Homework 3

Create a decorator that repeats function 5 times.

---

4. functools.wraps

When using decorators, function metadata is lost.

Example problem:

def decorator(func):
    def wrapper():
        return func()
    return wrapper


@decorator
def hello():
    "This is hello function"
    print("Hello")

print(hello.__name__)

Output:

wrapper

---

Fix using wraps

from functools import wraps

def decorator(func):
    @wraps(func)
    def wrapper():
        return func()
    return wrapper


@decorator
def hello():
    print("Hello")

print(hello.__name__)

Output:

hello

---

Why wraps is important

Without wraps	With wraps
wrapper name shown	original name preserved
loses docstring	keeps metadata

---

Homework 4

Apply wraps in a decorator and check function name.

---

5. Real-World Decorators

---

Logging Decorator

def log(func):
    def wrapper(*args, **kwargs):
        print("Function is running")
        return func(*args, **kwargs)
    return wrapper


@log
def add(a, b):
    return a + b

print(add(5, 3))

Output:

Function is running
8

---

Authentication Decorator

def login_required(func):
    def wrapper(user):
        if user == "admin":
            return func(user)
        return "Access Denied"
    return wrapper


@login_required
def dashboard(user):
    return "Welcome Admin"

print(dashboard("admin"))

---

Timing Decorator

import time

def timer(func):
    def wrapper():
        start = time.time()
        func()
        end = time.time()
        print("Time:", end - start)
    return wrapper

---

Real World Mapping

Decorator	Use Case
logging	system logs
auth	login check
timer	performance monitoring
retry	API retry logic

---

Summary Table

Concept	Meaning
Decorator	modifies function behavior
Chaining	multiple decorators
Arguments	parameterized decorators
wraps	preserves metadata

---

Decorator Flow Diagram

function
   ↓
decorator(func)
   ↓
wrapper()
   ↓
modified behavior

---

Homework 5 (Final Challenge)

Create a decorator that: - logs function name - logs arguments - logs result

---

Solution

def logger(func):
    def wrapper(*args, **kwargs):
        print("Function:", func.__name__)
        print("Args:", args)
        result = func(*args, **kwargs)
        print("Result:", result)
        return result
    return wrapper


@logger
def multiply(a, b):
    return a * b

multiply(4, 5)

---

Next Advanced Topics

• Generators
• Iterators
• Context Managers
• Async Programming (async/await)
• functools (partial, reduce, lru_cache)
• Metaclasses ```