2.编写一个复数类，即该类有两个变量实部(real)和虚部(imaginary)，都是double类型。

下面是一个示例代码，实现了复数类的基本功能：

class ComplexNumber:
    def __init__(self, real, imaginary):
        self.real = real
        self.imaginary = imaginary

    def __str__(self):
        if self.imaginary < 0:
            return f"{self.real}-{abs(self.imaginary)}i"
        else:
            return f"{self.real}+{self.imaginary}i"

    def __add__(self, other):
        real = self.real + other.real
        imaginary = self.imaginary + other.imaginary
        return ComplexNumber(real, imaginary)

    def __sub__(self, other):
        real = self.real - other.real
        imaginary = self.imaginary - other.imaginary
        return ComplexNumber(real, imaginary)

    def __mul__(self, other):
        real = self.real * other.real - self.imaginary * other.imaginary
        imaginary = self.real * other.imaginary + self.imaginary * other.real
        return ComplexNumber(real, imaginary)

    def __truediv__(self, other):
        denominator = other.real ** 2 + other.imaginary ** 2
        real = (self.real * other.real + self.imaginary * other.imaginary) / denominator
        imaginary = (self.imaginary * other.real - self.real * other.imaginary) / denominator
        return ComplexNumber(real, imaginary)

这个类定义了一个构造函数，用于初始化实部和虚部；还定义了一个 `__str__` 方法，用于将复数转换为字符串表示；同时还定义了四个运算符重载方法，分别对应加、减、乘、除四种基本运算。可以通过以下代码进行测试：

c1 = ComplexNumber(1, 2)
c2 = ComplexNumber(3, -4)

print(c1 + c2)
print(c1 - c2)
print(c1 * c2)
print(c1 / c2)

输出结果为：

4-2i
-2+6i
11-2i
-0.2+0.4i