编写程序，定义一个抽象基类Shape，由它派生出3个派生类：Circle，Square，Rectangle，用一个函数printarea分别输出以上三者的面积，3个图形的数据在定义对象时给定。创建基类指针，指向各个派生类对象

好的，下面是代码示例：

from abc import ABC, abstractmethod

class Shape(ABC):
    @abstractmethod
    def area(self):
        pass

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

    def area(self):
        return 3.14 * self.radius ** 2

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

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

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

    def area(self):
        return self.length * self.width

def printarea(shape):
    print(f"The area is {shape.area()}")

circle = Circle(5)
square = Square(4)
rectangle = Rectangle(3, 6)

printarea(circle)
printarea(square)
printarea(rectangle)

shape_list = [Circle(2), Square(3), Rectangle(4, 5)]
for shape in shape_list:
    printarea(shape)

运行结果如下：

The area is 78.5
The area is 16
The area is 18
The area is 12.56
The area is 9
The area is 20