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Python学习day25-面向对象之组合，多态和封装

组合

组合，简单来说就是对象的某个属性是另一个类的对象，有点类似于嵌套的意思，但是不全一样。使用组合最大的优点还是可以减少代码的冗余。举一个最简单的组合的例子：

xxxxxxxxxx

class Foo:

    def __init__(self,bar):

        self.bar = bar

class Bar:

    pass

f = Foo()

f.bar = Bar()

f = Foo(Bar())

上述例子中，类中的Foo的内置属性中就有Bar类，所以是组合的一种体现。

再来看一个稍微复杂一点的例子：

需求，我们需要一个老师类，一个学生类，学生类包括姓名，年龄，所选课程，

老师类包括姓名，年龄，所教课程。

运用组合的方法实现如下：

xxxxxxxxxx

# 定义一个人基类，老师类和学生类都从此继承出来

class Person:

school = 'oldboy'

def __init__(self, name, age):

self.name = name

self.age = age

# 定义老师类，包括姓名，年龄，所教课程

class Teacher(Person):

def __init__(self, name, age, course):

super().__init__(name, age)

self.course_list = course

# course是课程对象，是老师教授的课程

# 定义课程对象，课程包括名字，价格，时间周期

class Course:

def __init__(self, c_name, c_price, c_period):

self.name = c_name

self.price = c_price

self.period = c_period

# 定义学生类，继承于人基类，且和课程类有所组合

class Student(Person):

# course = list()#课程的列表不能放在这里，因为每次实例化都会清零，所以这是错误放法

def __init__(self, name, age):

super().__init__(name, age)

self.course_list = list()# 课程列表应该放在这里，也就是实例化时候默认加载的地方，init里面

# course是课程对象，是学生选择的课程

def choose_course(self, course):

# self.course = list()#课程同样不能放在这里，因为每次学生调用选课方法的话课程列表就会清零，错误放法

self.course_list.append(course)

# 定义一个学生查看自己所选课程的方法

def tell_all_course(self):

# 循环学生选课列表，每次拿出一个课程对象

for course in self.course_list:

print(course.name)

# 实例化产生一个课程对象

course = Course('Python', 20180, 7)

# 实例化产生一个学生对象

stu1 = Student('nick', 19)

# 学生对象调用选课方法，选择上面产生的课程对象

stu1.choose_course(course)

stu2 = Student('www', 19)

stu2.choose_course(course)

stu2.choose_course(Course('linux', 19999, 5))

stu2.tell_all_course()

# print(stu1.__dict__)

# print(stu2.__dict__)

# teacher = Teacher('tank', 19, '高级', course)

# 查看老师教授的课程名

# print(teacher.course.name)

# 这种连续调用的方法会极大的减少代码量，也会使整个的项目变得简练，逻辑清楚，即每个对象都是一个百宝箱。

多态与多态性

多态，顾名思义，多种形态，即一类事物的多种形态。

比如现实生活中，动物类，包括猪，狗，猫，长颈鹿，老虎等等，就是同一类事物的多重形态，当然我们还可以继续往下分类，这个问题在这里不是关键。

而多态性就是指在不考虑实例类型的情况下使用实例。

我们用最简单的实例来实现一下多态的概念。

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class Animal:

def speak(self):

pass

class Pig(Animal):

def speak(self):

print('哼哼哼')

class Dog(Animal):

def speak(self):

print('汪')

class People(Animal):

def speak(self):

print('hello')

pig = Pig()

dog = Dog()

people = People()

pig.speak()

dog.speak()

people.speak()

def animal_speak(obj):

obj.speak()

animal_speak(pig)

# 这个实例的意思大概就是，每个动物都能发出自己的声音，那么我们给每个动物定义一个类，以及同样的speak方法，那么我们就可以通过定义另外一个调用自身speak方法的函数来实现动物发出声音这个功能，只需要在animal_speak后面加上需要发声的动物就行了。

所以其实这种调用方式有一个问题，就是如果定义别的动物类的时候其内部没有speak这个方法，那么就会报错，所以我们就要想办法去约束这个事情，官方的话就是我们要约束子类的共有特性。

常用的约束方法有两种：

一个是用abc实现接口的统一化，abc和装饰器的用法有些类似，需要在前面加@

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import abc

class Animal(metaclass=abc.ABCMeta):

@abc.abstractclassmethod

def speak(self):  # 规定了我的子类必须有这个方法，如果没有，调用就没有输出，就不能利用多态性，做约束作用

pass

class Pig(Animal):

@abc.abstractclassmethod

def speak(self):

print('哼哼哼')

class Dog(Animal):

@abc.abstractclassmethod

def zz(self):

print('汪')

class People(Animal):

@abc.abstractclassmethod

def cc(self):

print('hello')

pig = Pig()

dog = Dog()

people = People()

pig.speak()

dog.speak()

people.speak()

def animal_speak(obj):

obj.speak()

animal_speak(pig)

第二个就是比较常用的方式，就是用异常处理来约束子类
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11

1

19

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class Animal():

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def speak(self):

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# 主动抛出异常

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raise Exception('需要重写')

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class Pig(Animal):

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def speak(self):

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print('哼哼哼')

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pig = Pig()

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def animal_speak(obj):

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obj.speak()

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animal_speak(pig)

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# 这种情况下，如果子类里面没有父类的方法，那么调用时候就会主动抛出异常，通知你需要重写这些方法。
- 说起来是有这两种约束子类的方法，但实际上，Python是一种十分崇尚鸭子类型的语言，鸭子类型就是说，只要你走起路来像鸭子，那么你就是鸭子。换做在Python里就是说，只要你能实现这些功能，我们不太关心你是怎么实现的，用了什么方法或者变量，所以一般Python做的一些比较大的项目，并不会去约束你的子类方法是如何写的，而只是靠程序员自己来把控，自由度非常高。

封装

封装的字面意思就是把一堆东西放在袋子里，然后封上口。但是如果封装==隐藏的话，就太片面了。

封装只是在于明确其使用范围，封装的属性或者方法，都可以在类内部直接使用，但是不能被外部直接使用，我们需要另外写一些函数或者方法，来让外部以我们想让他们看到的方式看到这些隐藏的内容。

我们先来看一下如何去把类里面的属性或方法设置成隐藏：

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# 隐藏属性：主要目的是为了保护原有数据，不让别人修改

# 把name隐藏起来

class Person:

def __init__(self, name, age):

self.__name = name# 在需要隐藏的属性前面加上__就可以将其设置成隐藏属性，也就是封装

self.age = age

def get_name(self):  # 可以另外增加一些功能，或者不同的打印方式

return self.__name

p = Person('nick', 18)

print(p.age)

# print(p.__name)

print(p.get_name())

# Python中隐藏的属性就真的访问不到了么，并不是的

# 实际上隐藏的属性还是可以访问到

print(p.__dict__)# 我们先查看对象的字典属性，里面会写真正的被隐藏变量的名字

print(p._Person__name)# 直接用我们看到的异常属性名字就可以直接看到隐藏变量

# 不过非常不推荐这种做法，毕竟隐藏属性就是为了隐藏起来，你非要去把人家调出来干嘛。。。

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# 隐藏方法：主要目的是隔离复杂度，使用者不用考虑有多少种方法，只需要调用那个对外的方法就可以了，毕竟里面的方法你也看不到

class Person:

def __init__(self, name, age):

self.__name = name

self.age = age

def __speak(self):  # 通过封装方法，让方法只在内部使用，对外只提供一个接口就可以

print('666')

p = Person('nick', 89)

# p._Person__speak()

print(Person.__dict__)