三个形状，一句话算完

三个形状：长方形 3×4、正方形边长 5、三角形底 6 高 5。三个类各自实现 area，调用方一句类型判断都没有：

class Shape:
    """只承诺一件事：能算面积"""
    def area(self):
        raise NotImplementedError

    def name(self):
        raise NotImplementedError


class Rect(Shape):
    def __init__(self, w, h):
        self.w, self.h = w, h

    def area(self):
        return self.w * self.h

    def name(self):
        return "长方形"


class Square(Shape):
    def __init__(self, s):
        self.s = s

    def area(self):
        return self.s ** 2

    def name(self):
        return "正方形"


class Tri(Shape):
    def __init__(self, b, h):
        self.b, self.h = b, h

    def area(self):
        return self.b * self.h // 2

    def name(self):
        return "三角形"


SHAPES = [Rect(3, 4), Square(5), Tri(6, 5)]

print("/".join(str(s.area()) for s in SHAPES))