Python编程：从入门到实践-类

• 1、创建类

#!/usr/bin/env python
#-*- encoding:utf-8 -*-
#dog.py
class Dog():
  """一个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() + " is now sitting.")
  def roll_over(self):
    """模拟小狗被命令时打滚"""
    print(self.name.title() + " rolled over!")

#方法__init__() ：类中的函数成为方法，开头和结尾各有两个下划线，旨在避免Python默认方法与普通方法发生名字冲突。方法__init__定义三个形参self、name和age

__init__方法定义两个以self为前缀的变量self.name、self.age，以self为前缀的变量可供类中的所有方法使用。self.name = name获取存储在形参name中的值，并存储

在变量name中。然后该变量被关联到当前创建的实例。这样通过实例访问的变量成为属性。

• 根据类创建实例

#!/usr/bin/env python
#-*- encoding:utf-8 -*-
#dog.py
class Dog():
  """一个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() + " is now sitting.")
  def roll_over(self):
    """模拟小狗被命令时打滚"""
    print(self.name.title() + " rolled over!")

my_dog = Dog('willie',6)
print("\nMy dog's name is " + my_dog.name.title() + ".")
print("\nMy dog's " + str(my_dog.age) + " years old.")

#访问属性 my_dog.name

#调用方法 

my_dog.sit()

my_dog.roll_over()

• 使用类和实例

#!/usr/bin/env python
#-*- encoding:utf-8 -*-
class Car():
  """一次模拟汽车的简单尝试"""
  def __init__(self, make, model, year):
    self.make = make
    self.model = model
    self.year = year
  def get_descriptive_name(self):
    """返回整洁的描述性信息"""
    long_name = str(self.year) + ' ' + self.make + ' ' + self.model
    return long_name.title()
my_new_car = Car('audi', 'a4', 2016)
print(my_new_car.get_descriptive_name())

• 给属性指定默认值

#!/usr/bin/env python
#-*- encoding:utf-8 -*-
class Car():
  """一次模拟汽车的简单尝试"""
  def __init__(self, make, model, year):
    self.make = make
    self.model = model
    self.year = year
    self.odometer_reading = 0
  def get_descriptive_name(self):
    """返回整洁的描述性信息"""
    long_name = str(self.year) + ' ' + self.make + ' ' + self.model
    return long_name.title()
  def read_odometer(self):
    """打印一条指出汽车里程的消息"""
    print("This car has " + str(self.odometer_reading) + " miles on it.")
my_new_car = Car('audi', 'a4', 2016)
print(my_new_car.get_descriptive_name())
my_new_car.read_odometer()

•  修改属性的值

#!/usr/bin/env python
#-*- encoding:utf-8 -*-
class Car():
  """一次模拟汽车的简单尝试"""
  def __init__(self, make, model, year):
    self.make = make
    self.model = model
    self.year = year
    self.odometer_reading = 0
  def get_descriptive_name(self):
    """返回整洁的描述性信息"""
    long_name = str(self.year) + ' ' + self.make + ' ' + self.model
    return long_name.title()
  def read_odometer(self):
    """打印一条指出汽车里程的消息"""
    print("This car has " + str(self.odometer_reading) + " miles on it.")
my_new_car = Car('audi', 'a4', 2016)
#print(my_new_car.get_descriptive_name())
my_new_car.odometer_reading = 23
my_new_car.read_odometer()

• 通过方法修改属性的值

#!/usr/bin/env python
#-*- encoding:utf-8 -*-
class Car():
  """一次模拟汽车的简单尝试"""
  def __init__(self, make, model, year):
    self.make = make
    self.model = model
    self.year = year
    self.odometer_reading = 0
  def get_descriptive_name(self):
    """返回整洁的描述性信息"""
    long_name = str(self.year) + ' ' + self.make + ' ' + self.model
    return long_name.title()
  def read_odometer(self):
    """打印一条指出汽车里程的消息"""
    print("This car has " + str(self.odometer_reading) + " miles on it.")
  def update_odometer(self, mileage):
    """将里程数读数设置为指定的值"""
    self.odometer_reading = mileage
my_new_car = Car('audi', 'a4', 2016)
#print(my_new_car.get_descriptive_name())
my_new_car.update_odometer(23)
my_new_car.read_odometer()

#!/usr/bin/env python
#-*- encoding:utf-8 -*-
class Car():
  """一次模拟汽车的简单尝试"""
  def __init__(self, make, model, year):
    self.make = make
    self.model = model
    self.year = year
    self.odometer_reading = 23
  def get_descriptive_name(self):
    """返回整洁的描述性信息"""
    long_name = str(self.year) + ' ' + self.make + ' ' + self.model
    return long_name.title()
  def read_odometer(self):
    """打印一条指出汽车里程的消息"""
    print("This car has " + str(self.odometer_reading) + " miles on it.")
  def update_odometer(self, mileage):
    """
    将里程表读数设置为指定的值
    禁止将里程表读数往回调
    """
    if mileage >= self.odometer_reading:
      self.odometer_reading = mileage
    else:
      print("You can't roll back an odometer!")
my_new_car = Car('audi', 'a4', 2016)
my_new_car.update_odometer(10)
my_new_car.read_odometer()

• 通过方法对属性的值进行递增

#!/usr/bin/env python
#-*- encoding: utf-8 -*-
class Car():
  """一次模拟汽车的简单尝试"""
  def __init__(self, make, model, year):
    """初始化描述汽车的属性"""
    self.make = make 
    self.model = model
    self.year = year
    self.odometer_reading = 0
  def get_descriptive_name(self):
    """返回整洁的描述性信息"""
    long_time = str(self.year) + ' ' + self.make + ' ' + self.model
    return long_time.title()
  def update_odometer(self, mileage):
    """将里程表读数设置为指定值，禁止将里程表读数往回调"""
    if mileage >= self.odometer_reading:
      self.odometer_reading = mileage
    else:
      print("You can't roll back an odometer!")
  def read_odometer(self):
    """打印一条指出汽车里程的消息"""
    print("This car has " + str(self.odometer_reading) + " miles on it. ")
  def increment_odometer(self, miles):
    """将里程表读数增加指定的量"""
    self.odometer_reading += miles
my_used_car = Car('subaru', 'outback', 2013)
print(my_used_car.get_descriptive_name())
my_used_car.update_odometer(23500)
my_used_car.read_odometer()
my_used_car.increment_odometer(100)
my_used_car.read_odometer()

• 2、继承

如果编写的类是另一个现成类的特殊版本，可使用继承。一个类集成另一个类时，它将自动获得另一个类的所有属性和方法；原有的类称为父类，而新类称为子类。子类继承了其父类的所有属性和方法，同时还可以定义自己的属性和方法。

• 子类的方法__init__()

Python2.7中的继承

#!/usr/bin/env python
#-*- encoding: utf-8 -*-
class Car():
  def __init__(self, make, model, year):
    --snip--
class ElectricCar(Car):
  def ___init__(self, make, model, year):
    super(ElectricCar, self).__init__(make, model, year)
    --snip--

• 子类定义属性和方法

#!/usr/bin/env python
#-*- encoding: utf-8 -*-
#electric_car.py
class Car():
  """一个模拟汽车的简单尝试"""
  def __init__(self, make, model, year):
    self.make = make
    self.model = model
    self.year = year
    self.odometer_reading = 0
  def get_descriptive_name(self):
    long_time = str(self.year) + ' ' + self.make + ' ' + self.model
    return long_time.title()
  def read_odometer(self):
    print("This car has " + str(self.odometer_reading) + " miles on it. ")
  def update_odemeter(self, mileage):
    if mileage >= self.odometer_reading:
      self.odometer_reading = mileage
    else:
      print("You can't roll back an odometer!")
  def increment_odometer(self, miles):
    self.odometer_reading += miles
class ElectricCar(Car):
  """电动汽车的独特之处,初始化父类的属性，再初始化电动汽车特有的属性"""
  def __init__(self, make, model, year):
    super().__init__(make, model, year)
    self.bettery_size = 70
  def describe_battery(self):
    """打印一条描述电瓶容量的消息"""
    print("This car has a " + str(self.bettery_size) + "-kWh battery.")
my_tesla = ElectricCar('tesla', 'model s', 2016)
print(my_tesla.get_descriptive_name())
my_tesla.describe_battery()

C:\PyProjects>C:/Python37/python3.exe c:/PyProjects/demo2.py
2016 Tesla Model S
This car has a 70-kWh battery.

　　子类添加了新属性self.battery_size(),并设置其初始值(70)，根据ElectricCar类创建的所有实例都将包含这个属性，但所有Car实例都不包含它。

• 重写父类的方法

 对于父类的方法，只要不符合子类模拟的事务的行为，都可对其进行重写。可在子类中定义一个这样的方法，即它与要重写的父类方法同名，这样，Python将不会考虑这个父类方法，而只关注在子类中定义的相应方法。

class ElectricCar(Car):
  ---snip---
  def fill_gas_tank():
    """电动汽车没有油箱"""
    print("This car doesn't need a gas tank!")

• 将实例用作属性

 使用代码模拟事务时，会发现自己给类添加的细节越来越多：属性和方法清单以及文件越来越长。在这种情况下，可能需要将类的一部分作为一个独立的类提取出来。将大型类拆分成多个协同工作的小类。

• 3、导入类

• 导入单个类

#!/usr/bin/env python
#-*- encoding: utf-8 -*-
class Car():
  """一个模拟汽车的简单尝试"""
  def __init__(self, make, model, year):
    self.make = make
    self.model = model
    self.year = year
    self.odometer_reading = 0
  def get_descriptive_name(self):
    long_time = str(self.year) + ' ' + self.make + ' ' + self.model
    return long_time.title()
  def read_odometer(self):
    print("This car has " + str(self.odometer_reading) + " miles on it. ")
  def update_odemeter(self, mileage):
    if mileage >= self.odometer_reading:
      self.odometer_reading = mileage
    else:
      print("You can't roll back an odometer!")
  def increment_odometer(self, miles):
    self.odometer_reading += miles

from car import Car

my_new_car = Car('audi', 'a4', 2016)
print(my_new_car.get_descriptive_name())
my_new_car.odometer_reading = 23
my_new_car.read_odometer()

C:\PyProjects>C:/Python37/python3.exe c:/PyProjects/demo3.py
2016 Audi A4
This car has 23 miles on it.