6、定义Circle类，要求：包括私有属性__radius，构造函数为半径赋值，构造函数的默认参数值为0，析构函数输出适当信息，普通方法SetRadius用于设置半径，普通方法Area返回圆面积，内置方法__str__用于输出圆面积，内置方法__gt__用于比较两个圆面积大小。并创建两个实例分别验证上述功能。

以下是Circle类的代码实现：

import math

class Circle:
    def __init__(self, radius=0):
        self.__radius = radius

    def __del__(self):
        print("Circle object is deleted")

    def SetRadius(self, radius):
        self.__radius = radius

    def Area(self):
        return math.pi * self.__radius ** 2

    def __str__(self):
        return "The area of the circle is {:.2f}".format(self.Area())

    def __gt__(self, other):
        return self.Area() > other.Area()

我们可以创建两个实例c1和c2，然后对它们进行测试：

c1 = Circle(5)
c2 = Circle()

print(c1.Area())  # 输出78.54
print(c2.Area())  # 输出0.00

c2.SetRadius(3)
print(c2.Area())  # 输出28.27

print(c1)  # 输出The area of the circle is 78.54
print(c2)  # 输出The area of the circle is 28.27

if c1 > c2:
    print("c1 is larger than c2")
else:
    print("c2 is larger than c1")

del c1
del c2

输出结果为：

78.53981633974483
0.0
28.274333882308138
The area of the circle is 78.54
The area of the circle is 28.27
c1 is larger than c2
Circle object is deleted
Circle object is deleted

可以看到，Circle类的所有要求均得到了满足，并且两个实例的创建、修改和删除都没有问题。