编程要求 本关的编程任务是，补全step1/step1.py文件中 Begin-End 区间的代码，完成指定三维向量类的功能。具体要求如下： 定义一个类 Vector3d，补全其中的代码，使其包含 3 个私有数据成员 x、y、z，并能够在构造方法中，接收3个参数，对这3个私有数据成员进行初始化； 具体输入输出请参见后续测试样例。

以下是补全后的代码：

class Vector3d:
    def __init__(self, x, y, z):
        self.__x = x
        self.__y = y
        self.__z = z

    def __str__(self):
        return "({}, {}, {})".format(self.__x, self.__y, self.__z)

    def __add__(self, other):
        return Vector3d(self.__x + other.__x, self.__y + other.__y, self.__z + other.__z)

    def __sub__(self, other):
        return Vector3d(self.__x - other.__x, self.__y - other.__y, self.__z - other.__z)

    def __mul__(self, other):
        if isinstance(other, Vector3d):
            return Vector3d(self.__y * other.__z - self.__z * other.__y,
                            self.__z * other.__x - self.__x * other.__z,
                            self.__x * other.__y - self.__y * other.__x)
        else:
            return Vector3d(self.__x * other, self.__y * other, self.__z * other)

    def __rmul__(self, other):
        return self.__mul__(other)

    def __truediv__(self, other):
        return Vector3d(self.__x / other, self.__y / other, self.__z / other)

    def __eq__(self, other):
        return self.__x == other.__x and self.__y == other.__y and self.__z == other.__z

    def __ne__(self, other):
        return not self.__eq__(other)

    def dot_product(self, other):
        return self.__x * other.__x + self.__y * other.__y + self.__z * other.__z

    def cross_product(self, other):
        return Vector3d(self.__y * other.__z - self.__z * other.__y,
                        self.__z * other.__x - self.__x * other.__z,
                        self.__x * other.__y - self.__y * other.__x)

    def length(self):
        return math.sqrt(self.__x ** 2 + self.__y ** 2 + self.__z ** 2)

    def normalize(self):
        l = self.length()
        if l == 0:
            return Vector3d(0, 0, 0)
        else:
            return Vector3d(self.__x / l, self.__y / l, self.__z / l)

其中，`__init__`方法用来初始化三维向量的坐标；`__str__`方法用来将三维向量转化为字符串输出；`__add__`、`__sub__`、`__mul__`、`__rmul__`、`__truediv__`方法实现三维向量的加、减、乘、除运算；`__eq__`和`__ne__`方法用来判断两个三维向量是否相等；`dot_product`方法用来计算两个三维向量的点积；`cross_product`方法用来计算两个三维向量的叉积；`length`方法用来计算三维向量的长度；`normalize`方法用来将三维向量转化为单位向量。