Python学习day24-面向对象的三大特征之继承

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Python学习day23-面向对象的三大特征之继承继承的介绍利用继承减少代码的冗余重用父类方法的两种方法第一种：第二种：

Python学习day24-面向对象的三大特征之继承

继承的介绍

继承，在现实生活中即是继承遗产等等，Python里概念也与之类似，继承是一种新建类的方式，继承了一个类，那么类中的属性和方法就在子类中了。

继承中我们通常会提到几个概念，即父类（也叫基类），子类（也叫派生类）。父类就是被继承的类，而子类就是继承父类的类。

新式类和经典类：

1. 新式类就是只要继承了object类，就是新式类，实际上在Python里默认继承的就是object类，所以Python3里面只有新式类，只有Python2里才有经典类。
2. 顾名思义，就是没有继承object类的类，Python2中定义新式类需要在括号里写上object。

利用继承减少代码的冗余

很明显，继承的优势之一就是可以极大减少代码的重复量，也就是我们在定义类的时候，如果两个类中有很多的属性定义方式是完全相同的，这时我们就可以另外定义一个基类，用这两个类去继承这个基类，从而减少代码量。比如以下例子：

 

 

 

x

 

 

 

 

1

class Teacher:

2

    school = 'xiaoxue'

3

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

4

        self.name = name

5

 self.age = age

6

 self.level = level

7

class Student:

8

school = 'xiaoxue'

9

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

10

        self.name = name

11

 self.age = age

12

 self.course = course

 

 

用继承改造之后，代码就是这个样子：

 

 

 

x

1

28

 

 

 

 

1

class Person():

2

 school = 'xiaoxue'

3

​

4

 def __init__(self, name, age):

5

 self.name = name

6

 self.age = age

7

​

8

​

9

class Teacher(Person):

10

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

11

        Person.__init__(name,age)# 这种重用方式我们将在下面介绍

12

self.level = level

13

​

14

class Student(Person):

15

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

16

        Percon.__init__(name,age)

17

        self.course = course

18

19

stu1 = Student('nick', 19)

20

stu1 = Student()  # 报错，因为父类中有__init__，所以必须传两个参数

21

stu1.school = 'xxx'

22

print(stu1.school)

 

 

也许看起来代码没有少多少，这是因为这里只有两个类调用这个基类，如果是一个比较大的项目，有数十甚至数百的类调用一个项目，那么代码量将会少太多。

 

 

 

xxxxxxxxxx

1

31

 

 

 

 

1

# 这里我们需要注意一下，关于属性的查找顺序的问题

2

# 正常的属性查找顺序就是

3

# 先找所在对象，然后找所属类，然后所属父类，最后报错

4

# 但还有一种不正常的顺序，就是菱形问题，这是一种非常复杂非常麻烦，但是偶尔会出现的问题，所以要注意，并掌握其解决办法,下面就是一个菱形问题的代码

5

class A(object):

6

    def test(self):

7

        print('from A')

8

​

9

class B(A):

10

    def test(self):

11

        print('from B')

12

​

13

class C(A):

14

    def test(self):

15

        print('from C')

16

​

17

class D(B):

18

    def test(self):

19

        print('from D')

20

​

21

class E(C):

22

    def test(self):

23

        print('from E')

24

​

25

class F(D,E):

26

    # def test(self):

27

    #     print('from F')

28

    pass

29

f1=F()

30

f1.test()

31

print(F.__mro__) #__mro__是查看当前类的一个线性链表，也就是查找的顺序，但是要注意只有新式类才有这个属性可以查看线性列表，经典类没有这个属性

 

 

新式类和经典类在查询放面的区别在于：

 

 

 

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1

 

 

 

 

1

新式类：广度优先--左侧开始，一直往上找，找到菱形的顶点结束，不包括顶点，继续下一个继承的父类往上找，如此循环，直到最后找到菱形顶点结束 

2

经典类：深度优先--左侧开始，一直往上找，找到菱形的顶点结束，包括顶点，然后继续下一个继承的父类往上寻找

 

 

重用父类方法的两种方法

第一种：

重用父类方法的第一种方法就是直接用父类的类名来调用其内部的属性或者方法，也可以叫做是指名道姓的方法，即父类名.父类方法（）。实际上这种重用方法和继承没什么关系，即便不继承，只要两个类在同一个文件中，父类在子类的上面，就可以直接这样使用。示例代码如下：

 

 

 

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1

35

 

 

 

 

1

class A:

2

def __init__(self, name, age):

3

self.name = name

4

self.age = age

5

​

6

​

7

class Person():

8

school = 'oldboy'

9

​

10

def __init__(self, name, age):

11

self.name = name

12

self.age = age

13

​

14

def study(self):

15

print('study....')

16

​

17

​

18

class Teacher(Person):

19

pass

20

​

21

​

22

class Student(Person):

23

# school = 'yyyy'

24

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

25

# 如何重用父类的__init__的方法

26

Person.__init__(self, name, age)# 这里就是指名道姓法

27

self.course = course

28

​

29

def study(self):

30

Person.study(self)

31

print(f'{self.name}在学习')

32

33

stu1 = Student('wed', 19, 'Python')

 

 

第二种：

第二种也就是我们日后最常用的一个方法，就是通过super()关键字来重用父类

 

 

 

x

 

 

 

 

1

class Person():

2

 school = 'oldboy'

3

 def __init__(self, name, age):

4

 self.name = name

5

 self.age = age

6

 def study(self):

7

 print('study....')

8

​

9

class Student(Person):

10

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

11

 # 如何重用父类的__init__的方法

12

 # super()会按照mro列表拿到父类对象

13

 # 对象调用绑定方法，不需要传递第一个参数

14

 super().__init__(name, age)

15

 # 经典类和新式类中super()的用法是有区别的

16

 # 经典类中必须按下面这种写，Python3中没有经典类，所以都用上面空括号的方式写

17

 # super(Student, self).__init__(name, age)#在Python3中为了兼容Python2的代码也可以这样写。

18

 self.course = course

19

​

20

 def study(self):

21

 super().study()

22

 print(f'{self.name}在学习')

23

​

24

​

25

 stu1 = Student('wed', 19, 'Python')

26

 stu1.study()

 

 

两种方法的适用场景不一样，具体差别如下：

1. 如果类与类之间有继承关系的话，一般会用super
2. 指名道姓的方式一般适用于两个类之间没有继承关系，而且如果是同时继承多个父类，想要调用其中某个父类的某个方法的话，还是用指名道姓会比较准确，因为super会按照mro列表顺序寻找的，效率低不说，如果不同类之间出现同样的变量名的话，很容易出错。