What is a decorator in Python and how do you write one?

A decorator is a function that takes another function as input, wraps it with extra behaviour, and returns the wrapped version. The @decorator syntax is shorthand for func = decorator(func). Decorators exploit the fact that Python functions are first-class objects.

import time

def timer(func):
    def wrapper(*args, **kwargs):
        start  = time.perf_counter()
        result = func(*args, **kwargs)      # call the original
        end    = time.perf_counter()
        print(f'{func.__name__} took {end-start:.4f}s')
        return result
    return wrapper

@timer
def compute(n):
    return sum(range(n))

compute(1_000_000)
# compute took 0.0312s

The problem with the naive version above is that wrapper.__name__ is 'wrapper', not 'compute', which confuses debuggers and documentation tools. Always apply functools.wraps(func) to the inner wrapper to preserve the original function's metadata:

from functools import wraps

def timer(func):
    @wraps(func)
    def wrapper(*args, **kwargs):
        ...
    return wrapper

Decorators can be stacked — @dec1 over @dec2 applies dec2 first, then dec1. Common built-in decorators: @staticmethod, @classmethod, @property, @functools.lru_cache (memoisation). In frameworks, @app.route in Flask and @pytest.fixture are decorator-based APIs.

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