__new__ quacks like __call__

class_as_decorator.py

class trace:
    lv = 0

    # https://docs.python.org/3/reference/datamodel.html#object.__new__
    def __new__(cls, fn):
        def wrapper(x):
            indent = 4 * cls.lv * ' '
            cls.lv += 1
            print(indent + f'f({x})')
            res = fn(x)
            print(indent + f'-> {res}')
            cls.lv -= 1
            return res
        return wrapper

# replace `lru_cache` with `cache` if you're using Python 3.9+
from functools import lru_cache

@trace
# @lru_cache
def fib(n):
    if n < 2: return n
    return fib(n-2) + fib(n-1)
    # return fib(n-1) + fib(n-2)

fib(4)
# fib(10)