使用类的封装性思想：声明复数类Complex,成员变量包括实部和虚部，成员方法包括实现由字符串构造复数，复数加法，减法，字符串描述，比较相等的操作

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

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

def __eq__(self, other):
return self.real == other.real and self.imaginary == other.imaginary

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

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

@classmethod
def from_string(cls, string):
real, imaginary = string.split("+")
real = float(real.strip())
imaginary = float(imaginary.strip()[:-1])
return cls(real, imaginary)