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 '''
