python 面向对象之多态

多态是什么？
　　用一句话来概括下，多态就是同一操作（方法）作用于不同的对象时，可以有不同的解释，产生不同的执行结果。

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# @Time    : 2017/12/26 0026 15:18
# @Author  : ming
class Person(object):
    def __init__(self, name, gender):
        self.name = name
        self.gender = gender

    def who(self):
        return 'I am a Person, my name is %s' % self.name

class Student(Person):
    def __init__(self, name, gender, score):
        super(Student, self).__init__(name, gender)
        self.score = score

    def who(self):
        return 'I am a Student, my name is %s' % self.name

class Teacher(Person):
    def __init__(self, name, gender, course):
        super(Teacher, self).__init__(name, gender)
        self.course = course

    def who(self):
        return 'I am a Teacher, my name is %s' % self.name

def run(x):
    print(x.who())

a = Person("keke", "Male")
b = Student("Coke", "Male", 98)
c = Teacher("Jone", "Female", "Chinese")

run(a)
run(b)
run(c)

  　在一个函数中，如果我们接收一个变量 x，则无论该 x 是 Person、Student还是 Teacher，都可以正确打印出结果：

I am a Person, my name is keke
I am a Student, my name is Coke
I am a Teacher, my name is Jone

　　这种行为称为多态。也就是说，方法调用将作用在 x 的实际类型上。s 是Student类型，它实际上拥有自己的 who()方法以及从 Person继承的 who方法，但调用 s.who()总是先查找它自身的定义，如果没有定义，则顺着继承链向上查找，直到在某个父类中找到为止。

　　由于Python是动态语言，所以，传递给函数 who(x)的参数 x 不一定是 Person 或 Person 的子类型。任何数据类型的实例都可以，只要它有一个who()的方法即可。

　　在继承关系中，如果一个实例的数据类型是某个子类，那它的数据类型也可以被看做是父类。但是，反过来就不行，如下：

#!/usr/bin/env python
# -*- coding: utf-8 -*-
# @Time    : 2017/12/26 0026 15:18
# @Author  : ming

class Animal:
    def run(self):
        print('Animal is running...')

class Dog(Animal):
    def run(self):
        print('Dog is running...')

class Cat(Animal):
    def run(self):
        print('Cat is running...')

def run_twice(animal):
    animal.run()
    animal.run()

a = Animal()
d = Dog()
c = Cat()

print('a is Animal?', isinstance(a, Animal))
print('a is Dog?', isinstance(a, Dog))
print('a is Cat?', isinstance(a, Cat))

print('d is Animal?', isinstance(d, Animal))
print('d is Dog?', isinstance(d, Dog))
print('d is Cat?', isinstance(d, Cat))

run_twice(c)

a is Animal? True
a is Dog? False
a is Cat? False
d is Animal? True
d is Dog? True
d is Cat? False
Cat is running...
Cat is running...