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02.面向对象/03.类的继承.md
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02.面向对象/03.类的继承.md
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# 类的继承
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## 面向对象的继承
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不用继承创建对象
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```python
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class Person:
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def __init__(self,name,sex,age):
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self.name = name
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self.age = age
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self.sex = sex
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class Cat:
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def __init__(self,name,sex,age):
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self.name = name
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self.age = age
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self.sex = sex
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class Dog:
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def __init__(self,name,sex,age):
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self.name = name
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self.age = age
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self.sex = sex
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```
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使用继承的方式
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```python
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class Aniaml(object):
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def __init__(self,name,sex,age):
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self.name = name
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self.age = age
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self.sex = sex
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class Person(Aniaml):
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pass
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class Cat(Aniaml):
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pass
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class Dog(Aniaml):
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pass
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```
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继承的有点也是显而易见的：
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1. 增加了类的耦合性（耦合性不宜多，宜精）。
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2. 减少了重复代码。
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3. 使得代码更加规范化，合理化。
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## 继承的分类
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上面的那个例子：
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- Aminal 叫做父类,基类,超类。
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- Person Cat Dog: 子类，派生类。
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继承：可以分**单继承，多继承**。
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这里需要补充一下python中类的种类（继承需要）：
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在python2x版本中存在两种类.：
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- ⼀个叫**经典类**. 在python2.2之前. ⼀直使⽤的是经典类. 经典类在基类的根如果什么都不写.
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- ⼀个叫**新式类**. 在python2.2之后出现了新式类. 新式类的特点是基类的根是object类。
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python3x版本中只有一种类：
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python3中使⽤的都是**新式类**. 如果基类谁都不继承. 那这个类会默认继承 object
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## 单继承
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### 类名，对象执行父类方法
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```python
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class Aniaml(object):
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type_name = '动物类'
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def __init__(self,name,sex,age):
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self.name = name
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self.age = age
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self.sex = sex
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def eat(self):
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print('吃',self)
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class Person(Aniaml):
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pass
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class Cat(Aniaml):
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pass
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class Dog(Aniaml):
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pass
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print(Person.type_name)
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Person.eat('东西')
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print(Person.type_name)
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p1 = Person('aaron','男',18)
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print(p1.__dict__)
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print(p1.type_name)
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p1.type_name = '666'
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print(p1)
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p1.eat()
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```
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### 执行顺序
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```python
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class Aniaml(object):
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type_name = '动物类'
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def __init__(self,name,sex,age):
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self.name = name
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self.age = age
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self.sex = sex
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def eat(self):
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print('吃',self)
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class Person(Aniaml):
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def eat(self):
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print('%s 用筷子吃饭'%self.name)
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class Cat(Aniaml):
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pass
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class Dog(Aniaml):
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pass
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p1 = Person('eagle','男',18)
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p1.eat()
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```
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### 同时执行类以及父类方法
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方法一：如果想执行父类的func方法，这个方法并且子类中夜用，那么就在子类的方法中写上：父类.func(对象,其他参数)
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```python
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class Aniaml(object):
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type_name = '动物类'
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def __init__(self,name,sex,age):
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self.name = name
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self.age = age
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self.sex = sex
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def eat(self):
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print('吃东西')
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class Person(Aniaml):
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def __init__(self,name,sex,age,mind):
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Aniaml.__init__(self,name,sex,age)
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self.mind = mind
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def eat(self):
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Aniaml.eat(111)
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print('%s 吃饭'%self.name)
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class Cat(Aniaml):
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pass
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class Dog(Aniaml):
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pass
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p1 = Person('aaron','男',18,'想吃东西')
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p1.eat()
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```
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方法二：利用super，super().func(参数)
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```python
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class Aniaml(object):
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type_name = '动物类'
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def __init__(self,name,sex,age):
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self.name = name
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self.age = age
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self.sex = sex
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def eat(self):
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print('吃东西')
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class Person(Aniaml):
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def __init__(self,name,sex,age,mind):
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# super(Person,self).__init__(name,sex,age)
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super().__init__(name,sex,age)
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self.mind = mind
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def eat(self):
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super().eat()
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print('%s 吃饭'%self.name)
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class Cat(Aniaml):
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pass
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class Dog(Aniaml):
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pass
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p1 = Person('aaron','男',18,'想吃东西')
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p1.eat()
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```
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单继承练习题
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```python
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class Base:
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def __init__(self,num):
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self.num = num
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def func1(self):
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print(self.num)
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class Foo(Base):
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pass
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obj = Foo(123)
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obj.func1()
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# 运⾏的是Base中的func1
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```
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```python
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class Base:
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def __init__(self,num):
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self.num = num
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def func1(self):
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print(self.num)
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class Foo(Base):
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def func1(self):
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print("Foo.func1",self.num)
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obj = Foo(123)
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obj.func1()
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# 运⾏的是Foo中的func1
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```
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```python
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class Base:
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def __init__(self, num):
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self.num = num
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def func1(self):
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print(self.num)
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self.func2()
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def func2(self):
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print("Base.func2")
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class Foo(Base):
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def func2(self):
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print("Foo.func2")
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obj = Foo(123)
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obj.func1()
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# func1是Base中的 func2是⼦类中的
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```
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```python
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class Base:
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def __init__(self, num):
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self.num = num
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def func1(self):
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print(self.num)
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self.func2()
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def func2(self):
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print(111, self.num)
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class Foo(Base):
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def func2(self):
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print(222, self.num)
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lst = [Base(1), Base(2), Foo(3)]
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for obj in lst:
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obj.func2()
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```
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```python
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class Base:
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def __init__(self, num):
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self.num = num
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def func1(self):
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print(self.num)
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self.func2()
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def func2(self):
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print(111, self.num)
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class Foo(Base):
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def func2(self):
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print(222, self.num)
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lst = [Base(1), Base(2), Foo(3)]
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for obj in lst:
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obj.func1()
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```
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## 多继承
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```python
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class ShenXian: # 神仙
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def fei(self):
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print("神仙都会⻜")
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class Monkey: # 猴
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def chitao(self):
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print("猴⼦喜欢吃桃⼦")
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class SunWukong(ShenXian, Monkey): # 孙悟空是神仙, 同时也是⼀只猴
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pass
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sxz = SunWukong() # 孙悟空
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sxz.chitao() # 会吃桃⼦
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sxz.fei() # 会⻜
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```
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### 经典类的多继承
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```python
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class A:
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pass
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class B(A):
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pass
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class C(A):
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pass
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class D(B, C):
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pass
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class E:
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pass
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class F(D, E):
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pass
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class G(F, D):
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pass
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class H:
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pass
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class Foo(H, G):
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pass
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```
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画图
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在经典类中采⽤的是深度优先，遍历⽅案. 什么是深度优先. 就是⼀条路走到头. 然后再回来. 继续找下⼀个.
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类的MRO(method resolution order): Foo-> H -> G -> F -> E -> D -> B -> A -> C.
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### 新式类的多继承
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#### mro序列
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MRO是一个有序列表L，在类被创建时就计算出来。
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通用计算公式为：
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mro(Child(Base1，Base2)) = [ Child ] + merge( mro(Base1), mro(Base2), [ Base1, Base2] )（其中Child继承自Base1, Base2）
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如果继承至一个基类：class B(A)
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这时B的mro序列为
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mro( B ) = mro( B(A) )
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= [B] + merge( mro(A) + [A] )
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= [B] + merge( [A] + [A] )
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= [B,A]
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如果继承至多个基类：class B(A1, A2, A3 …)
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这时B的mro序列
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mro(B) = mro( B(A1, A2, A3 …) )
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= [B] + merge( mro(A1), mro(A2), mro(A3) ..., [A1, A2, A3] )
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= ...
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计算结果为列表，列表中至少有一个元素即类自己，如上述示例[A1,A2,A3]。merge操作是C3算法的核心。
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#### 表头和表尾
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表头：列表的第一个元素
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表尾：列表中表头以外的元素集合（可以为空）
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示例：列表：[A, B, C] 表头是A，表尾是B和C
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#### 列表之间的+操作
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[A] + [B] = [A, B]
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merge操作示例：
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如计算merge( [E,O], [C,E,F,O], [C] )
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有三个列表 ： ① ② ③
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1 merge不为空，取出第一个列表列表①的表头E，进行判断
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各个列表的表尾分别是[O], [E,F,O]，E在这些表尾的集合中，因而跳过当前当前列表
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2 取出列表②的表头C，进行判断
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C不在各个列表的集合中，因而将C拿出到merge外，并从所有表头删除
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merge( [E,O], [C,E,F,O], [C]) = [C] + merge( [E,O], [E,F,O] )
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3 进行下一次新的merge操作 ......
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---------------------
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计算mro(A)方式：
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mro(A) = mro( A(B,C) )
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原式= [A] + merge( mro(B),mro(C),[B,C] )
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mro(B) = mro( B(D,E) )
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= [B] + merge( mro(D), mro(E), [D,E] ) # 多继承
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= [B] + merge( [D,O] , [E,O] , [D,E] ) # 单继承mro(D(O))=[D,O]
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= [B,D] + merge( [O] , [E,O] , [E] ) # 拿出并删除D
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= [B,D,E] + merge([O] , [O])
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= [B,D,E,O]
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mro(C) = mro( C(E,F) )
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= [C] + merge( mro(E), mro(F), [E,F] )
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= [C] + merge( [E,O] , [F,O] , [E,F] )
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= [C,E] + merge( [O] , [F,O] , [F] ) # 跳过O，拿出并删除
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= [C,E,F] + merge([O] , [O])
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= [C,E,F,O]
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原式= [A] + merge( [B,D,E,O], [C,E,F,O], [B,C])
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= [A,B] + merge( [D,E,O], [C,E,F,O], [C])
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= [A,B,D] + merge( [E,O], [C,E,F,O], [C]) # 跳过E
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= [A,B,D,C] + merge([E,O], [E,F,O])
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= [A,B,D,C,E] + merge([O], [F,O]) # 跳过O
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= [A,B,D,C,E,F] + merge([O], [O])
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= [A,B,D,C,E,F,O]
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那既然python提供了. 为什么我们还要如此⿇烦的计算MRO呢? 因为笔
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试.......你在笔试的时候, 是没有电脑的. 所以这个算法要知道. 并且简单的计算要会. 正式项⽬
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开发的时候很少有⼈这么去写代码.
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