《Python编程从入门到实践》_第九章_类

创建一个简单的类

根据Dog类创建的每个实列都将存储名字和年龄。我们赋予了每条小狗蹲下(sit())和打滚(roll_over())的能力：

 class Dog():
     """一次模拟小狗的简单尝试"""
     def __init__(self, name, age):
         """初始化属性name和age"""
         self.name = name
         self.age = age
     def sit(self):
         """模拟小狗被命令时蹲下"""
         print(self.name.title() + "now is sitting.")
     def roll_over(self):
         """模拟小狗被命令时打滚"""
         print(self.name.title() + "rolled over!")
 my_dog = Dog('tom','')
 print("my dog name is " + my_dog.name.title() )

根据约定，在Python中，首字母大写的名称指的是类：类中的函数称为方法

方法__init__()，开头和末尾都有2个下划线，这是一种约定，旨在避免Python默认方法与普通方法发生名称的冲突。我们将方法__init__()定义成了包含三个形参： self、 name和age。在这个方法的定义中，参self必不可少，还必须位于其他形参的前面。为何必须在方法定义中包含形参self呢？因为Python调用这个__init__()方法来创建Dog实例时，将自动传入实参self。 两个变量self_name和self_age都有前缀self，以self为前缀的变量都可供类中的所有方法使用，我们还可以通过类的任何实列来访问这些变量。像这样可通过实例访问的变量称为属性。

在python2.X中，如果创建类需要在括号后面加上(object)。

 

访问属性

继续上面的例子，方法__init__()创建一个表示特定小狗的示例，并使用我们提供的值来设置属性name和age，方法__init__()并未显式地包含return语句，但python自动返回一个表示这条小狗的示例。我们将这个示例存储在变量my_dog中。

class Dog():
    """一次模拟小狗的简单尝试"""
    def __init__(self, name, age):
        """初始化属性name和age"""
        self.name = name
        self.age = age
    def sit(self):
        """模拟小狗被命令时蹲下"""
        print(self.name.title() + " now is sitting.")
    def roll_over(self):
        """模拟小狗被命令时打滚"""
        print(self.name.title() + " rolled over!")
my_dog = Dog('tom',3)
print(my_dog.name)
print(my_dog.age)
#运行结果
tom
3

调用方法

class Dog():
    """一次模拟小狗的简单尝试"""
    def __init__(self, name, age):
        """初始化属性name和age"""
        self.name = name
        self.age = age
    def sit(self):
        """模拟小狗被命令时蹲下"""
        print(self.name.title() + " now is sitting.")
    def roll_over(self):
        """模拟小狗被命令时打滚"""
        print(self.name.title() + " rolled over!")
my_dog = Dog('tom',3)
my_dog.sit()
my_dog.roll_over()
#运行结果
Tom now is sitting.
Tom rolled over!

根据Dog类创建实例后，就可以使用句点表示来调用Dog来定义的任何方法

创建多个实例

class Dog():
    """一次模拟小狗的简单尝试"""
    def __init__(self, name, age):
        """初始化属性name和age"""
        self.name = name
        self.age = age
    def sit(self):
        """模拟小狗被命令时蹲下"""
        print(self.name.title() + " now is sitting.")
    def roll_over(self):
        """模拟小狗被命令时打滚"""
        print(self.name.title() + " rolled over!")
my_dog = Dog('tom',3)
your_dog = Dog('Mei',2)
print("My dog name is " + my_dog.name.title())
print("Your dog name is " + your_dog.name.title())
#运行结果
My dog name is Tom
Your dog name is Mei

可按需求根据类创建任意数量的实例。

使用类和实例

给属性指定默认值

类中的每个属性都必须有初始值，哪怕这个值是0或空字符串，在有些情况下，如设置默认值时，在方法__init__()内指定这种初始值时可以的没如果你对某个属性这样做了，就无需包含为他提供初始化的形参。

class Car():
    """一次模拟汽车的简单尝试"""
    def __init__(self, make, model, year):
        """汽车的初始化"""
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 100
    def get_descri_name(self):
        """描述汽车"""
        long_name = str(self.year) + ' ' + self.model + ' ' + self.make
        return long_name
my_car = Car('audi', 'a4', '')
print(my_car.model)
print(my_car.get_descri_name())
#运行结果
a4
2017 a4 audi

直接修改属性的值

class Car():
    """一次模拟汽车的简单尝试"""
    def __init__(self, make, model, year):
        """汽车的初始化"""
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 100
    def get_descri_name(self):
        """描述汽车"""
        long_name = str(self.year) + ' ' + self.model + ' ' + self.make
        return long_name
my_car = Car('audi', 'a4', '')
print(my_car.get_descri_name())
my_car.year = 2016
print(my_car.get_descri_name())
#运行结果
2017 a4 audi
2016 a4 audi

通过方法修改

class Car():
    """一次模拟汽车的简单尝试"""
    def __init__(self, make, model, year):
        """汽车的初始化"""
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 100
    def get_descri_name(self):
        """描述汽车"""
        long_name = str(self.year) + ' ' + self.model + ' ' + self.make
        return long_name
    def update(self, mile):
        """更新里程值"""
        if mile > self.odometer_reading:
            self.odometer_reading = mile
        else:
            print("You can't roll back an odometer")
    def increment_odometer(self,mile):
        """增加里程"""
        self.odometer_reading += mile
    def read_odometer(self):
        """打印汽车的里程"""
        print("This car has " + str(self.odometer_reading) + " miles on it.")
my_car = Car('audi', 'a4', '')
my_car.read_odometer()
my_car.odometer_reading = 10    #直接修改里程值
my_car.update(200)     #通过方法修改里程
my_car.read_odometer()
my_car.increment_odometer(10)
my_car.read_odometer()
#运行结果
This car has 100 miles on it.
This car has 200 miles on it.
This car has 210 miles on it.

继承

如果我们想再一个class继承另一个类的属性，可以在类后面括号中加入类的名称，举例如下：

class Car():
    """一次模拟汽车的简单尝试"""
    def __init__(self, make, model, year):
        """汽车的初始化"""
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 100
    def get_descri_name(self):
        """描述汽车"""
        long_name = str(self.year) + ' ' + self.model + ' ' + self.make
        return long_name
    def update(self, mile):
        """更新里程值"""
        if mile > self.odometer_reading:
            self.odometer_reading = mile
        else:
            print("You can't roll back an odometer")
    def increment_odometer(self,mile):
        """增加里程"""
        self.odometer_reading += mile
    def read_odometer(self):
        """打印汽车的里程"""
        print("This car has " + str(self.odometer_reading) + " miles on it.")
class ElectricCar(Car):
    """电动汽车的独特特性"""
    def __init__(self, make, model, year):
        """初始化父类的属性"""
        super().__init__(make, model, year)
my_tesla = ElectricCar('tesla', 'model s', '')
print(my_tesla.get_descri_name())
#运行结果
2016 model s tesla

为了继承父类的属性，还需要加入一个特殊的函数super()，帮助python将夫类和子类关联起来。

在python2.X中，类supper的格式如下：supper(Eletric,self).__init__(make, model, year)

给子类定义属性和方法

让一个类继承另一个类后，可添加区分子类和父类所属的新属性和新方法。

将实例用作属性

class Car():
    """一次模拟汽车的简单尝试"""
    def __init__(self, make, model, year):
        """汽车的初始化"""
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 100
    def get_descri_name(self):
        """描述汽车"""
        long_name = str(self.year) + ' ' + self.model + ' ' + self.make
        return long_name
    def update(self, mile):
        """更新里程值"""
        if mile > self.odometer_reading:
            self.odometer_reading = mile
        else:
            print("You can't roll back an odometer")
    def increment_odometer(self,mile):
        """增加里程"""
        self.odometer_reading += mile
    def read_odometer(self):
        """打印汽车的里程"""
        print("This car has " + str(self.odometer_reading) + " miles on it.")
class Battery():
    """一次模拟电动汽车"""
    def __init__(self,battery_size=70):
        """初始化电瓶的属性"""
        self.battery_size = battery_size
    def describe_battery(self):
        """打印一条描述电瓶容量的消息"""
        print("This car has a " + str(self.battery_size) + "-kwh battery.")
class ElectricCar(Car):
    """电动汽车的独特特性"""
    def __init__(self, make, model, year):
        """初始化父类的属性"""
        super().__init__(make, model, year)
        self.battery = Battery()
my_tesla = ElectricCar('tesla', 'model s', '')
print(my_tesla.get_descri_name())
my_tesla.battery.describe_battery()
#运行结果
2016 model s tesla
This car has a 70-kwh battery.

导入类

导入单个或者多个类

一个文件car.py

class Car():
    """一次模拟汽车的简单尝试"""
    def __init__(self, make, model, year):
        """汽车的初始化"""
        self.make = make
        self.model = model
        self.year = year
        self.odometer_reading = 100
    def get_descri_name(self):
        """描述汽车"""
        long_name = str(self.year) + ' ' + self.model + ' ' + self.make
        return long_name
    def update(self, mile):
        """更新里程值"""
        if mile > self.odometer_reading:
            self.odometer_reading = mile
        else:
            print("You can't roll back an odometer")
    def increment_odometer(self,mile):
        """增加里程"""
        self.odometer_reading += mile
    def read_odometer(self):
        """打印汽车的里程"""
        print("This car has " + str(self.odometer_reading) + " miles on it.")

class Battery():
    """一次模拟电动汽车"""
    def __init__(self,battery_size=70):
        """初始化电瓶的属性"""
        self.battery_size = battery_size
    def describe_battery(self):
        """打印一条描述电瓶容量的消息"""
        print("This car has a " + str(self.battery_size) + "-kwh battery.")
class ElectricCar(Car):
    """电动汽车的独特特性"""
    def __init__(self, make, model, year):
        """初始化父类的属性"""
        super().__init__(make, model, year)
        self.battery = Battery()

创建另一个文件my_car.py，导入一个类

from  car import Car
my_car = Car('audi', 'a4', '')

一个模块中可以存储多个类，所以可以一次导入多个类

from car import Car,Battery,ElectricCar
my_tesla = ElectricCar('tesla', 'model s', '')
print(my_tesla.get_descri_name())
my_tesla.battery.describe_battery()

导入整个模块

import car     #导入整个模块的时候，需要使用句点表示法访问需要的类
my_tesla = car.ElectricCar('tesla', 'model s', '')
print(my_tesla.battery)

导入所有类

from car import *    #导入所有的类