Python Decorators

# Python Decorators

Welcome to Chapter 21! Decorators let you modify or enhance functions without changing their source code. They're used extensively in frameworks like Flask, Django, and FastAPI.

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1. Learning Objectives #

• Understand functions as first-class objects.

• Create basic decorators.

• Use @decorator syntax.

• Chain multiple decorators.

• Build practical decorators.

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2. Functions as First-Class Objects #

```python id="py21ex1" # Functions can be assigned to variables def greet(name): return f"Hello, {name}!"

sayhello = greet # Assign function to variable print(say_hello("Alice")) # Hello, Alice!

# Functions can be passed as arguments def apply(func, value): return func(value)

print(apply(len, "Python")) # 6

# Functions can return functions def multiplier(factor): def multiply(x): return x * factor return multiply

double = multiplier(2) print(double(5)) # 10

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## 3. Basic Decorator

python id="py21ex2" def mydecorator(func): def wrapper(*args, kwargs): print("⏳ Before function call") result = func(*args, kwargs) print("✅ After function call") return result return wrapper

@mydecorator # Same as: greet = mydecorator(greet) def greet(name): print(f"Hello, {name}!")

greet("Alice") # Output: # ⏳ Before function call # Hello, Alice! # ✅ After function call

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## 4. Practical Decorators

### Timer Decorator

python id="py21ex3" import time

def timer(func): def wrapper(*args, kwargs): start = time.time() result = func(*args, kwargs) elapsed = time.time() - start print(f"⏱️ {func.name} took {elapsed:.4f}s") return result return wrapper

@timer def slowfunction(): time.sleep(1) return "Done!"

slow_function()

### Logger Decorator

python id="py21ex4" from datetime import datetime

def logger(func): def wrapper(*args, kwargs): timestamp = datetime.now().strftime("%H:%M:%S") print(f"[{timestamp}] Calling {func.name}({args}, {kwargs})") result = func(*args, kwargs) print(f"[{timestamp}] {func.name_} returned {result}") return result return wrapper

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

add(3, 5)

### Authentication Decorator

python id="py21ex5" def requireauth(func): def wrapper(user, *args, kwargs): if not user.get("isauthenticated"): print("❌ Access denied! Please login.") return None return func(user, *args, kwargs) return wrapper

@requireauth def viewdashboard(user): print(f"Welcome to dashboard, {user['name']}! 🎉")

viewdashboard({"name": "Alice", "isauthenticated": True}) viewdashboard({"name": "Bob", "is_authenticated": False})

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## 5. Decorators with Arguments

python id="py21ex6" def repeat(times): def decorator(func): def wrapper(*args, kwargs): for in range(times): result = func(*args, kwargs) return result return wrapper return decorator

@repeat(3) def sayhello(): print("Hello!")

sayhello() # Prints "Hello!" three times

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## 6. Preserving Function Metadata

python id="py21ex7" from functools import wraps

def mydecorator(func): @wraps(func) # Preserves original function's name and docstring def wrapper(*args, kwargs): return func(*args, kwargs) return wrapper

@mydecorator def greet(name): """Greet a user by name.""" return f"Hello, {name}!"

print(greet.name) # greet (not 'wrapper') print(greet.doc_) # Greet a user by name.

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## 7. Chaining Decorators

python id="py21_ex8" def bold(func): @wraps(func) def wrapper(*args, kwargs): return f"<b>{func(*args, kwargs)}</b>" return wrapper

def italic(func): @wraps(func) def wrapper(*args, kwargs): return f"<i>{func(*args, kwargs)}</i>" return wrapper

@bold @italic def greet(name): return f"Hello, {name}!"

print(greet("Alice")) # <b><i>Hello, Alice!</i></b> # Applied bottom-up: italic first, then bold

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## 8. Class-based Decorators

python id="py21ex9" class CountCalls: def init(self, func): self.func = func self.count = 0 def call(self, *args, kwargs): self.count += 1 print(f"📊 {self.func.name} called {self.count} times") return self.func(*args, kwargs)

@CountCalls def sayhi(): print("Hi!")

sayhi() # called 1 times sayhi() # called 2 times sayhi() # called 3 times ``

Q7: Class-based decorators use: A) init and call B) str C) iter D) get Answer: A

Q10: Without @wraps, func.name returns: A) Original name B) 'wrapper' C) None D) Error Answer: B

1. 2. Why use @wraps? Preserves the original function's name, doc, and other metadata.

1. 3. Real-world decorator examples? @app.route() (Flask), @loginrequired (Django), @property, @staticmethod, @classmethod.

• Use @decorator syntax for clean application.

• Always use @functools.wraps` to preserve metadata.

• Decorators can accept arguments (triple-nested pattern).

• Multiple decorators can be chained (applied bottom-up).

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12. Next Chapter Recommendation #

In Chapter 22: Working with Dates and Time, you'll master the datetime module for time-based operations! 🚀