How do decorators work? Write a decorator that logs how long a function takes.

Answer

• Functions are objects: you can pass them around and return them.
• @deco above def f means f = deco(f).
• The decorator returns a wrapper that runs code before/after calling the original.
• Use functools.wraps to keep the original name and docstring.
• With arguments you add a factory level: @retry(3) means f = retry(3)(f).

import functools, time

def timed(func):
    @functools.wraps(func)
    def wrapper(*args, **kwargs):
        start = time.perf_counter()
        try:
            return func(*args, **kwargs)
        finally:
            ms = (time.perf_counter() - start) * 1000
            print(f"{func.__name__} took {ms:.1f} ms")
    return wrapper

Key terms

• Decorator
• Higher-order function
• Closure
• functools.wraps
• finally