組合：一個物件的屬性值是另外一個類的物件
比如：alex.weapon 是 Weapon類的物件 ---> alex.weapon.hand18

練習

from math import pi
class Circle:
    def __init__(self,r):
        self.r = r
    def area(self):
        return self.r**2 * pi
    def perimeter(self):
        return 2*pi*self.r

class Ring:
    def __init__
 
(self,outside_r,inside_r):
        self.outside_c = Circle(outside_r) # 屬性值是另外一個類的物件
        self.inside_c = Circle(inside_r)
    def area(self):
        return self.outside_c.area() - self.inside_c.area()
    def perimeter(self):
        return self.outside_c.perimeter()+self.inside_c.perimeter()