继承与派生

一、继承

1.什么是继承

继承是一种新建类的方式，新建的类称之为子类或派生类，继承的父类称之为基类或超类。

在python中，一个子类可以继承多个父类

在其他语言中，一个子类只能继承一个父类

2.继承的作用

减少代码的冗余

3.继承的实现

先确认谁是父类，先抽象，再继承；谁是子类

# 父类
class Father1:
    x = 1
    pass

class Father2:
    pass

class Father3:
    pass


# 子类
class Sub(Father1):
    pass

# 子类查看父类__bases__
print(Sub.__bases__)
# (<class '__main__.Father1'>,)
print(Sub.x)
# 1

# 问题: 代码冗余
# 老师类
class OldboyTeacher:
    school = 'oldboy'
    country = 'China'

    def __init__(self, name, age, sex):
        self.name = name
        self.age = age
        self.sex = sex

    # 老师修改分数
    def change_score(self):
        print(f'老师 {self.name} 正在修改分数...')


# 学生类
class OldboyStudent:
    school = 'oldboy'
    country = 'China'

    def __init__(self, name, age, sex):
        self.name = name
        self.age = age
        self.sex = sex

    # 学生选择课程
    def choose_course(self):
        print(f'学生 {self.name} 正在选择课程...')


stu1 = OldboyStudent('YJG', 50, 'female')
print(stu1.school, stu1.name, stu1.age, stu1.sex)

tea1 = OldboyTeacher('tank', 17, 'male')
print(tea1.school, tea1.name, tea1.age, tea1.sex)

解决代码冗余

class People:
    school = 'oldboy'
    country = 'China'

    def __init__(self, name, age, sex):
        self.name = name
        self.age = age
        self.sex = sex

class Student(People):
    # school = 'oldboy'
    # country = 'China'

    # def __init__(self, name, age, sex):
    #     self.name = name
    #     self.age = age
    #     self.sex = sex
    def func(self):
        print('from student')

class Teacher(People):
    # school = 'oldboy'
    # country = 'China'

    # def __init__(self, name, age, sex):
    #     self.name = name
    #     self.age = age
    #     self.sex = sex
    def func(self):
        print('from teacher')


s1 = Student('张三', 23, '男')
t1 = Teacher('李四', 25, '女')

print(s1.name, s1.age, s1.sex, s1.school)
print(t1.name, t1.age, t1.sex, t1.school)

4.继承的查找

注意：程序的执行顺序是由上到下，父类必须定义在子类的上方

在继承背景下，对象属性的查找顺序:
　　1.先从对象自己的名称空间中查找
　　2.对象中没有，从子类的名称空间中查找。
　　3.子类中没有, 从父类的名称空间中查找，若父类没有，则会报错!

# 父类
class Father:
    x = 10
    pass
# 子类
class Sub(Father):
    x = 20
    pass
sub_obj = Sub()

# 此方法并未修改调用对象类的值，而是给调用产生的对象增加一个属性
print('对象的名称空间：', sub_obj.__dict__)
sub_obj.x = 30
print(sub_obj.x)      # 30
print('父类的名称空间：', Father.__dict__)
print('子类的名称空间：', Sub.__dict__)
print('对象的名称空间：', sub_obj.__dict__)

# 父类
class Father:
    x = 10
    pass
# 子类
class Sub(Father):
    x = 20
    pass
sub_obj = Sub()
# sub_obj.x = 30
print(sub_obj.x)        # 20


# 父类
class Father:
    x = 10
    pass
# 子类
class Sub(Father):
    # x = 20
    pass
sub_obj = Sub()
# sub_obj.x = 30
print(sub_obj.x)        # 10


# 父类
class Father:
    # x = 10
    pass
# 子类
class Sub(Father):
    # x = 20
    pass
sub_obj = Sub()
# sub_obj.x = 30
print(sub_obj.x)        # 报错

二、派生

1.什么是派生

指的是子类继承父类的属性与方法，并且派生出自己独有的属性与方法。
若子类中的方法名与父类的相同，优先用子类的。

2.派生的实现

子类继承父类，派生出自己的属性与方法，并且重用父类的属性与方法。

# 父类
class Foo:
    def f1(self):
        print('from Foo.f1')
    def f2(self):
        print('from Foo.f2')
        self.f1()
class Bar(Foo):
    # 重写
    def f1(self):
        print('from Bar.f1')

    def func(self):
        print('from Bar.func')
bar_obj = Bar()

bar_obj.f1()
# from Bar.f1
bar_obj.func()
# from Bar.func
bar_obj.f2()
"""
输出结果：
    from Foo.f2
    from Bar.f1
"""

问题：子类重写父类的__init__导致代码更加冗余

class OldboyPeople:
    school = 'oldboy'

    def __init__(self, name, age, sex):
        self.name = name
        self.age = age
        self.sex = sex


class OldboyTeacher(OldboyPeople):

    def __init__(self, name, age, sex, sal):
        self.name = name
        self.age = age
        self.sex = sex
        self.sal = sal

    def change_score(self):
        print(f'老师 {self.name} 修改分数...')


class OldboyStudent(OldboyPeople):

    def __init__(self, name, age, sex, girl):
        self.name = name
        self.age = age
        self.sex = sex
        self.girl = girl

    def choose_course(self):
        print(f'学生 {self.name} 选择课程...')


tea1 = OldboyTeacher('张三', 23, 'male', 1500)
stu1 = OldboyStudent('李四', 28, 'male', '凤姐')
print(tea1.name, tea1.age, tea1.sex, tea1.sal)
print(stu1.name, stu1.age, stu1.sex, stu1.girl)

解决问题: 子类重用父类的属性，并派生出新的属性。
两种方式：
　　1.直接引用父类的__init__为其传参，并添加子类的属性。
　　2.通过super来指向父类的属性。
　　　　- super()是一个特殊的类，调用super得到一个对象，该对象指向父类的名称空间。

注意: 使用哪一种都可以，但不能两种方式混合使用。

# 方法一：
class OldboyPeople:
    school = 'oldboy'

    def __init__(self, name, age, sex):
        self.name = name
        self.age = age
        self.sex = sex


class OldboyTeacher(OldboyPeople):

    def __init__(self, name, age, sex, sal):
        OldboyPeople.__init__(self, name, age, sex)
        self.sal = sal

    #     self.name = name
    #     self.age = age
    #     self.sex = sex
    #     self.sal = sal
    def change_score(self):
        print(f'老师 {self.name} 修改分数...')


class OldboyStudent(OldboyPeople):

    def __init__(self, name, age, sex, girl):
        OldboyPeople.__init__(self, name, age, sex)
        self.girl = girl
        # self.name = name
        # self.age = age
        # self.sex = sex
        # self.girl = girl

    def choose_course(self):
        print(f'学生 {self.name} 选择课程...')


tea1 = OldboyTeacher('张三', 23, 'male', 1500)
stu1 = OldboyStudent('李四', 28, 'male', '凤姐')
print(tea1.name, tea1.age, tea1.sex, tea1.sal)
print(stu1.name, stu1.age, stu1.sex, stu1.girl)

# 方法二
class OldboyPeople:
    school = 'oldboy'

    def __init__(self, name, age, sex):
        self.name = name
        self.age = age
        self.sex = sex


class OldboyTeacher(OldboyPeople):

    def __init__(self, name, age, sex, sal):
        super().__init__(name, age, sex)
        # self.name = name
        # self.age = age
        # self.sex = sex
        self.sal = sal

    def change_score(self):
        print(f'老师 {self.name} 修改分数...')


class OldboyStudent(OldboyPeople):

    def __init__(self, name, age, sex, girl):
        super().__init__(name, age, sex)
        # self.name = name
        # self.age = age
        # self.sex = sex
        self.girl = girl

    def choose_course(self):
        print(f'学生 {self.name} 选择课程...')


tea1 = OldboyTeacher('张三', 23, 'male', 1500)
stu1 = OldboyStudent('李四', 28, 'male', '凤姐')
print(tea1.name, tea1.age, tea1.sex, tea1.sal)
print(stu1.name, stu1.age, stu1.sex, stu1.girl)

3、新式类与经典类

- 新式类:
　　1.凡是继承object的类或子孙类都是新式类。
　　2.在python3中所有的类都默认继承object。

- 经典类:
　　1.在python2中才会有经典类与新式类之分。
　　2.在python2中，凡是没有继承object的类，都是经典类。

mro的查找顺序：

新式类：广度优先

经典类：深度优先

查找顺序可以从下图看出