Decorators: count the calls

A decorator is a function that takes a function and returns a new one that wraps it — adding behavior around the original without touching its body. The @ syntax is just sugar for f = decorator(f):

def announce(fn):
    def wrapper(*args, **kwargs):
        print("calling", fn.__name__)
        return fn(*args, **kwargs)
    return wrapper

@announce
def add(a, b):
    return a + b

add(2, 3)        # prints "calling add", returns 5

The inner wrapper closes over fn, so it can run extra code and still forward every argument with *args, **kwargs.

Your task

Write a decorator count_calls(fn) that wraps fn so the wrapper still returns whatever fn returns, but also exposes a .calls attribute on the wrapper holding how many times it has been invoked (starting at 0).

@count_calls
def square(n):
    return n * n

square(2)        # 4
square(5)        # 25
square.calls     # 2

Hint: define wrapper(*args, **kwargs) inside count_calls, increment a counter stored on the wrapper before calling fn, and set wrapper.calls = 0 before you return wrapper.