Home / Python / Day 7: Intermediate Python / Decorators

Decorators

Decorators modify or enhance functions without changing their source code.

Decorators

A decorator is a function that wraps another function, adding behavior before/after execution. They use the @syntax sugar.

Syntax

<pre><code># Basic decorator
def my_decorator(func):
    def wrapper(*args, **kwargs):
        print("Before function call")
        result = func(*args, **kwargs)
        print("After function call")
        return result
    return wrapper

@my_decorator
def say_hello(name):
    print(f"Hello, {name}!")

say_hello("Alice")
# Before function call
# Hello, Alice!
# After function call

# Timer decorator
import time
from functools import wraps

def timer(func):
    @wraps(func)  # Preserves function metadata
    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 slow_function():
    time.sleep(0.1)

slow_function()
</code></pre>

Revision Notes

• @decorator syntax sugar for func = decorator(func)
• Wrapper function adds behavior before/after
• *args, **kwargs pass through all arguments
• @functools.wraps preserves __name__, __doc__
• Decorators are used for logging, timing, auth, caching

Build a Logging Decorator

Hard

Write a decorator called log_call that prints "Calling <function_name>" before the decorated function runs, and "Finished <function_name>" after it returns. Apply it to a function greet(name) that returns f"Hello, {name}!".

Input:

greet("Alice")

Output:

Calling greet Finished greet Hello, Alice!

Show Hint

A decorator is a function that takes a function and returns a wrapper function. Use func.__name__ to get the function's name, and make sure the wrapper returns the original function's result.

Solve this Challenge

Show Solution

def log_call(func):
    def wrapper(*args, **kwargs):
        print(f"Calling {func.__name__}")
        result = func(*args, **kwargs)
        print(f"Finished {func.__name__}")
        return result
    return wrapper

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

print(greet("Alice"))

Comprehensions Back to Course

Decorators

Decorators

Syntax

Revision Notes

Decorators Quiz

Build a Logging Decorator