Python报错：cannot unpack non-iterable builtin_func

Python报错：

TypeError: cannot unpack non-iterable builtin_function_or_method object

我写了一个方法，去保存未知的变量，它仅适用于一个方程式，当我想基于一个运算符返回值时却无效。

生效的代码：

def solveforx(y):
    x, add, num1, equal, num2 = y.split()

    # convert the strings into ints
    num1, num2 = int(num1), int(num2)
    # convert the result into a string and join it to the string "x = "
    return "x = "+ str(num2 - num1)

print(solveforx("X + 5 = 9"))

报错的代码：

def solveforx(y):
    x, op, num1, equal, num2 = y.split

    num1, num2 = int(num1), int(num2)

    if op == "+":
        return "x = " + str(num2 - num1)
    elif op == "-":
        return "x = " + str(num1 + num2)
    elif op == "*":
        return "x = " + str(num2/num1)
    elif op == "/":
        return "x = " + str(num1*num2)
    else:
        return "wrong operator"

print(solveforx("X + 5 = 9"))

报错信息：

TypeError: cannot unpack non-iterable builtin_function_or_method object