Python 学习笔记15 类 - 继承

我们在编程的过程中，并非都是要重头开始。比如其他人已经有现成的类，我们可以使用其他找人编写的类。术语称之为： 继承。

当一个类继承例外一个类时，它可以获得这个类的所有属性和方法：原有的类称之为 父类，新的类称之为子类。子类可以继承父类的所有方法和属性，还可以自定一些自己的方法和属性。

比如我们已经有了一个叫汽车的父类，我们可以继承这个类，生成一个电动车的子类：

#-*- coding:utf-8 -*-

class Car():

    def __init__(self, make, model, year):
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 0

    def get_description_name(self):
        long_name = str(self.year) + ' ' + self.make + ' ' + self.model
        return long_name.title()

    def read_odometer(self):
        print("This car has " + str(self.odometer_reading) + " miles on it.")

    def update_odometer(self, mileage):
        if mileage >= self.odometer_reading:
            self.odometer_reading = mileage
        else:
            print("You cannot do that.")

    def increase_odometer(self, miles):
        if miles >= 0:
            self.odometer_reading += miles
        else:
            print("The value is invalid, please input the number which should more than zero.")

'''继承car，生成一个新类'''
class ElectricCar(Car):
    def __init__(self, make, model, year):
        super().__init__(make, model, year)

my_BYD = ElectricCar("BYD", "Tang", 2019)
print(my_BYD.get_description_name())

'''
输出:
2019 Byd Tang
'''

通过上面的代码，我们看到，我们基于一个car的父类，生成了一个ElectricCar的子类。

在类定义是，在括号里面包含父类的名称，来表示继承这个类： class NewClass（SupperClass）。

而真正继承父类的方法和属性的，则是在__init__方法中的super（）方法的使用，该方法告诉Python使用父类的__init__方法，来重新构造一个类。

通过上面的例子，我们可以看到，子类可以正确的调用父类的方法，实际上这时已经是子类的方法了。

我们也可以根据累的特性，给子类定义自己特有的属性和方法：

比如电动车有一个电瓶，并且有方法可以实时的显示当前的电量。

#-*- coding:utf-8 -*-

class Car():

    def __init__(self, make, model, year):
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 0

    def get_description_name(self):
        long_name = str(self.year) + ' ' + self.make + ' ' + self.model
        return long_name.title()

    def read_odometer(self):
        print("This car has " + str(self.odometer_reading) + " miles on it.")

    def update_odometer(self, mileage):
        if mileage >= self.odometer_reading:
            self.odometer_reading = mileage
        else:
            print("You cannot do that.")

    def increase_odometer(self, miles):
        if miles >= 0:
            self.odometer_reading += miles
        else:
            print("The value is invalid, please input the number which should more than zero.")

'''继承car，生成一个新类'''
class ElectricCar(Car):
    def __init__(self, make, model, year):
        super().__init__(make, model, year)
        self.battery_size = 100

    def describe_battery(self):
        print("Catr has " + str(self.battery_size) + "-kwh battery. " )

my_BYD = ElectricCar("BYD", "Tang", 2019)
print(my_BYD.get_description_name())
my_BYD.describe_battery()

'''
输出:
2019 Byd Tang
Catr has 100-kwh battery.
'''

在上述代码中，我们可以看到，我们在__init__方法中，添加了一个电瓶容量的属性，

self.battery_size = 100

并且添加了一个电动车特有的显示电量的方法。

    def describe_battery(self):
        print("Catr has " + str(self.battery_size) + "-kwh battery. " )

这些方法是属于子类（ElectricCar）的，它能够正确的被运行。

当父类中的某些方法，并不适用子类的时候怎么办呐？我们可以在子类中重新定义该方法。

比如Car类中有加汽油的方法，而这对电动车并不适用，我们可以在子类中对这个方法进行覆盖重写。子类在调用这个方法时，将采用子类的定义：

#-*- coding:utf-8 -*-

class Car():

    def __init__(self, make, model, year):
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 0

    def get_description_name(self):
        long_name = str(self.year) + ' ' + self.make + ' ' + self.model
        return long_name.title()

    def read_odometer(self):
        print("This car has " + str(self.odometer_reading) + " miles on it.")

    def update_odometer(self, mileage):
        if mileage >= self.odometer_reading:
            self.odometer_reading = mileage
        else:
            print("You cannot do that.")

    def increase_odometer(self, miles):
        if miles >= 0:
            self.odometer_reading += miles
        else:
            print("The value is invalid, please input the number which should more than zero.")

    def fill_gas(self):
        print("Car is filling gas.")

'''继承car，生成一个新类'''
class ElectricCar(Car):
    def __init__(self, make, model, year):
        super().__init__(make, model, year)
        self.battery_size = 100

    def describe_battery(self):
        print("Catr has " + str(self.battery_size) + "-kwh battery. " )

    def fill_gas(self):
        print("Electric car no gas tank.")

my_BYD = ElectricCar("BYD", "Tang", 2019)
my_BYD.fill_gas()

'''
输出:
Electric car no gas tank.
'''

我们在编写代码时候，需要灵活的对类进行定义。在编程思想中，现实生活中的所有对象，都可以被定义成类。

我们尽可能多订一些类，以简化我们的代码长度，同时也变成程序代码的维护和修改。

比如在上述例子中，我们对电动车类增加了一个电池的属性和相关的方法。其实我们也可以新建一个电池的类，将电池特有的属性和方法独立开来。这样我们可以根据这个类生成各式各样的实例：

#-*- coding:utf-8 -*-

class Car():

    def __init__(self, make, model, year):
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 0

    def get_description_name(self):
        long_name = str(self.year) + ' ' + self.make + ' ' + self.model
        return long_name.title()

    def read_odometer(self):
        print("This car has " + str(self.odometer_reading) + " miles on it.")

    def update_odometer(self, mileage):
        if mileage >= self.odometer_reading:
            self.odometer_reading = mileage
        else:
            print("You cannot do that.")

    def increase_odometer(self, miles):
        if miles >= 0:
            self.odometer_reading += miles
        else:
            print("The value is invalid, please input the number which should more than zero.")

    def fill_gas(self):
        print("Car is filling gas.")

'''生成一个电池类'''
class Battery():
    def __init__(self, size = 100):
        self.size = size

    def describe_battery(self):
        print("Battery has " + str(self.size) + "-kwh battery. " )

'''继承car，生成一个新类'''
class ElectricCar(Car):
    def __init__(self, make, model, year):
        super().__init__(make, model, year)
        self.battery = Battery()

    def fill_gas(self):
        print("Electric car no gas tank.")

my_BYD = ElectricCar("BYD", "Tang", 2019)
my_BYD.battery.describe_battery()

'''
输出:
Battery has 100-kwh battery.
'''

我么可以看到我们增加了一个电池类Battery()，该类有自己属性 size和方法describe_battery。我们在定义电动车时，增加了一个battery的属性，这个属性是一个baterry的实例，我们可以认为该属性实际上是一个对象 object，我们可以操作和使用它的属性和方法。

这样做的好处就是，有关电池的属性和方法的修改，可以放在battery类中进行处理。EelctricCar类中，只关注与其相关的属性和方法。比如我们可以添加一个电池能跑多少里程的方法，该方法与电池的容量相关：

#-*- coding:utf-8 -*-

class Car():

    def __init__(self, make, model, year):
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 0

    def get_description_name(self):
        long_name = str(self.year) + ' ' + self.make + ' ' + self.model
        return long_name.title()

    def read_odometer(self):
        print("This car has " + str(self.odometer_reading) + " miles on it.")

    def update_odometer(self, mileage):
        if mileage >= self.odometer_reading:
            self.odometer_reading = mileage
        else:
            print("You cannot do that.")

    def increase_odometer(self, miles):
        if miles >= 0:
            self.odometer_reading += miles
        else:
            print("The value is invalid, please input the number which should more than zero.")

    def fill_gas(self):
        print("Car is filling gas.")

'''生成一个电池类'''
class Battery():
    def __init__(self, size = 100):
        self.size = size

    def describe_battery(self):
        print("Battery has " + str(self.size) + "-kwh battery. " )

    def show_range(self):
        print("Battery has " + str(self.size * 3) + " killmaters on full charge")

'''继承car，生成一个新类'''
class ElectricCar(Car):
    def __init__(self, make, model, year):
        super().__init__(make, model, year)
        self.battery = Battery()

    def fill_gas(self):
        print("Electric car no gas tank.")

my_BYD = ElectricCar("BYD", "Tang", 2019)

my_BYD.battery.describe_battery()
my_BYD.battery.show_range()
my_BYD.battery.size = 200
my_BYD.battery.describe_battery()
my_BYD.battery.show_range()
'''
输出:
Battery has 100-kwh battery.
Battery has 300 killmaters on full charge
Battery has 200-kwh battery.
Battery has 600 killmaters on full charge
'''