Python · Decorators

Functions are objects. They can be passed around, returned, and kept inside other functions (closures). A decorator is a function that takes a function and returns a new one, usually a wrapper that adds behaviour before or after the original runs.

@timer
def f(): ...

is exactly the same as f = timer(f).

• Use functools.wraps on the wrapper. Otherwise the decorated function loses its __name__ and docstring.
• A decorator that takes arguments (@repeat(3)) needs one extra layer: a function that returns a decorator.
• The standard library already ships useful ones, for example functools.lru_cache.

import functools, time

def timer(fn):
    @functools.wraps(fn)
    def wrapper(*args, **kwargs):
        t0 = time.perf_counter()
        result = fn(*args, **kwargs)
        ms = (time.perf_counter() - t0) * 1000
        print(f"{fn.__name__} took {'<1' if ms < 1 else round(ms)} ms")
        return result
    return wrapper

def repeat(n):
    def deco(fn):
        @functools.wraps(fn)
        def wrapper(*a, **k):
            return [fn(*a, **k) for _ in range(n)]
        return wrapper
    return deco

@timer
def add(a, b):
    """Add two numbers."""
    return a + b

@repeat(3)
def hello(name):
    return f"hi {name}"

print(add(2, 3), add.__name__, add.__doc__)
print(hello("Sahaj"))

@functools.lru_cache(maxsize=None)
def fib(n):
    return n if n < 2 else fib(n - 1) + fib(n - 2)
print(fib(80), fib.cache_info().hits)

Output:

add took <1 ms
5 add Add two numbers.
['hi Sahaj', 'hi Sahaj', 'hi Sahaj']
23416728348467685 78