def trace(func):
    """A decorator that accepts the same arguments
    and returns the same value as FUNC, but also
    prints the arguments and return value."""
    def afunc(*args):
        print("Call", func.__name__, "with", args)
        v = func(*args)
        print(func.__name__, "returns", v)
        return v
    return afunc

@trace
def sum_squares(k):
    if k <= 0:
        return 0
    else:
        return k*k + sum_squares(k-1)

sum_squares(3)

def check_result(checker):
    """A decorator that applies the predicate CHECKER to
    the result of the decorated function and raises an error
    if the check fails."""
    def checked_func(func):
        def call_and_check(*args):
            result = func(*args)
            if checker(result):
                return result
            else:
                raise ValueError("bad result") # indicate an error
        return call_and_check
    return checked_func

@check_result(lambda x: x < 1000)
def limited_id(y):
    return y

print(limited_id(5))
print(limited_id(1000))   # Raises exception