Python Decorators: A Beginner's Guide

📌 Quick Info

• Topic: Decorators in Python
• Target Audience: Beginners who know functions and want to level up
• Goal: Demystify one of Python's most misunderstood features

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1. Introduction

"Decorators looked like magic the first time I saw them. The @ symbol, a function wrapping another function... none of it made sense. Then I learned they're just functions calling functions. Here's how they actually work."

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2. The Problem (Repeating Code)

def greet():
    print("Starting...")
    print("Hello!")
    print("Finishing...")

def farewell():
    print("Starting...")
    print("Goodbye!")
    print("Finishing...")

Problem: The “Starting….” and “Finishing….” lines are repeated. If you want to change them, you edit every function.

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3. The Solution (Decorators)

def log_before_and_after(func):
    def wrapper():
        print("Starting...")
        func()
        print("Finishing...")
    return wrapper

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

@log_before_and_after
def farewell():
    print("Goodbye!")

greet()

Output:

Starting...
Hello!
Finishing...

The @ symbol applies the decorator to the function- now greet automatically logs before and after.

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4. How It Works (Step by Step)

• Step 1: log_before_and_after takes a function as input.
• Step 2: Inside, it defines wraper()- a new function that runs extra code before and after calling func().
• Step 3: It returns wrapper.
• Step 4: @log_before_and_after above greet is the same as writing greet= log_before_and_after(greet).

That’s the whole trick. Decorators are just functions that take functions and return functions.

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5. Real Example (Timing)

import time

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

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

print(slow_function())

Output:

slow_function took 1.0023s
Done

Now you can time any function by adding @timer above it. No changes to the function itself.

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6. Why args* and **kwargs?

Notice wrapper(*args, **kwargs) in the example above. That's what makes the decorator work with any function — no matter how many arguments it takes. Without it, the decorator only works on functions with no arguments.

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7. Common Use Cases

Use Case	What It Does
Logging	Print when a function starts and ends
Timing	Measure how long a function takes
Catching	Store Results so you don’t recompute
Authentication	Check permissions before running a function.
Retry Logic	Try again if the function fails

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8. What I learned

• A decorator is a function that wraps another function.
• @decorator is shorthand for func= decorator (func).
• **args, *kwargs makes a decorator work with any function.
• Decorators are everywhere in real code- flask, Django, pytest all use them.
• Start simple- one decorator that logs is enough to understand the pattern.

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9. Conclusion

"Decorators looked like wizardry until I understood they're just functions calling functions. Now I use them for logging and timing everywhere. The @ symbol isn't magic — it's a shortcut.”