用python编写代码，定义一个形状基类，有2个属性：面积和周长，以及两个无返回值的方法：area()和perimeter()，分别计算形状的面积和周长，从基类派生出三个子类：三角形、矩形、圆，重载基类的两个方法。

以下是代码实现：

import math

class Shape:
    def __init__(self):
        self.area_val = 0
        self.perimeter_val = 0

    def area(self):
        pass

    def perimeter(self):
        pass

class Triangle(Shape):
    def __init__(self, a, b, c):
        super().__init__()
        self.a = a
        self.b = b
        self.c = c

    def area(self):
        s = (self.a + self.b + self.c) / 2
        self.area_val = math.sqrt(s * (s - self.a) * (s - self.b) * (s - self.c))
        return self.area_val

    def perimeter(self):
        self.perimeter_val = self.a + self.b + self.c
        return self.perimeter_val

class Rectangle(Shape):
    def __init__(self, width, height):
        super().__init__()
        self.width = width
        self.height = height

    def area(self):
        self.area_val = self.width * self.height
        return self.area_val

    def perimeter(self):
        self.perimeter_val = 2 * (self.width + self.height)
        return self.perimeter_val

class Circle(Shape):
    def __init__(self, radius):
        super().__init__()
        self.radius = radius

    def area(self):
        self.area_val = math.pi * self.radius ** 2
        return self.area_val

    def perimeter(self):
        self.perimeter_val = 2 * math.pi * self.radius
        return self.perimeter_val

接下来可以用以下代码进行测试：

t = Triangle(3, 4, 5)
print("Triangle area:", t.area())
print("Triangle perimeter:", t.perimeter())

r = Rectangle(2, 3)
print("Rectangle area:", r.area())
print("Rectangle perimeter:", r.perimeter())

c = Circle(5)
print("Circle area:", c.area())
print("Circle perimeter:", c.perimeter())

输出：

Triangle area: 6.0
Triangle perimeter: 12
Rectangle area: 6
Rectangle perimeter: 10
Circle area: 78.53981633974483
Circle perimeter: 31.41592653589793