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Python面向对象编程之旅

OOP编程是什么

大家好,作为小白,最近学习了很多Python OOP编程的知识,因为脑容量有限,特此一一按照学习顺序记录下来,如果哪里有错误,还请大神尽快指出,以免误导他人。。。

首先让我们简单了解一下何为面向对象编程:

把一组数据结构和处理它们的方法组成对象(object),把相同行为的对象归纳为类(class),通过类的封装(encapsulation)隐藏内部细节,通过继承(inheritance)实现类的特化(specialization)和泛化(generalization),通过多态(polymorphism)实现基于对象类型的动态分派。

这样一说貌似有些复杂,简单来看的话可以参考下面的解释:

常见概念一览

概念 解释
类(Class) 用来描述具有相同的属性和方法的对象的集合。它定义了该集合中每个对象所共有的属性和方法。对象是类的实例
类变量 类变量在整个实例化的对象中是公用的。类变量定义在类中且在函数体之外。类变量通常不作为实例变量使用
数据成员 类变量或者实例变量, 用于处理类及其实例对象的相关的数据
方法重写 如果从父类继承的方法不能满足子类的需求,可以对其进行改写,这个过程叫方法的覆盖(override),也称为方法的重写
局部变量 定义在方法中的变量,只作用于当前实例的类
实例变量 在类的声明中,属性是用变量来表示的。这种变量就称为实例变量,是在类声明的内部但是在类的其他成员方法之外声明的
继承 即一个派生类(derived class)继承基类(base class)的字段和方法。继承也允许把一个派生类的对象作为一个基类对象对待。例如,有这样一个设计:一个Dog类型的对象派生自Animal类,这是模拟"是一个(is-a)"关系(例图,Dog是一个Animal)
实例化 创建一个类的实例,类的具体对象
方法 类中定义的函数
对象 通过类定义的数据结构实例。对象包括两个数据成员(类变量和实例变量)和方法

定义一个类

下面让我们简单定义一个汽车类:

class Car:
    def __init__(self, color, model, year):

        self.color = color
        self.model = model
        self.year = year

这里我们创建了一个汽车类Car,它有三个公共属性,分别是color(颜色),model(型号),year(生产年份)

创建实例对象,访问属性

现在让我们新建一个对象my_car:

my_car = Car("yellow", "beetle", 1967)

查看一下my_car的属性

print(f" My {my_car.color} car {my_car.model} is made in {my_car.year}")
# My yellow car beetle is made in 1967

添加新属性

我们想要给my_car添加一个新属性wheels

my_car.wheels = 5
print(f"Wheels: {my_car.wheels}")
# Wheels: 5

使用dir(my_car)可以让我们确认一下属性是否存在:

dir(my_car)

Out:
['__class__',
 '__delattr__',
 '__dict__',
 '__dir__',
 '__doc__',
 '__eq__',
 '__format__',
 '__ge__',
 '__getattribute__',
 '__gt__',
 '__hash__',
 '__init__',
 '__init_subclass__',
 '__le__',
 '__lt__',
 '__module__',
 '__ne__',
 '__new__',
 '__reduce__',
 '__reduce_ex__',
 '__repr__',
 '__setattr__',
 '__sizeof__',
 '__str__',
 '__subclasshook__',
 '__weakref__',
 'color',
 'model',
 'wheels',     <====已经添加成功啦
 'year']

类变量,修改类变量的值

在Python中,我们在类外声明一个类变量,下面让我们修改一下Car类:

class Car:
    wheels = 0
    def __init__(self, color, model, year):
        self.color = color
        self.model = model
        self.year = year

这样的话,我们在调用wheels这个变量时,可以通过实例,或者直接调用Car.wheels:

my_car = Car("yellow", "beetle", 1967)
print(f"My car is {my_car.color}")
print(f"It has {Car.wheels} wheels")
print(f"It has {my_car.wheels} wheels")

Out:
My car is yellow
It has 0 wheels
It has 0 wheels

这里需要注意一下,如果想要通过my_car.wheels =xxx来修改wheels的值,不会真正修改类变量wheels的值,我们来看一个具体的例子:

my_car = Car("yellow", "Beetle", "1966")
my_other_car = Car("red", "corvette", "1999")

print(f"My car is {my_car.color}")
print(f"It has {my_car.wheels} wheels")


print(f"My other car is {my_other_car.color}")
print(f"It has {my_other_car.wheels} wheels")

Out:
My car is yellow
It has 0 wheels
My other car is red
It has 0 wheels

我们首先创建两个实例my_car 和my_other_car ,默认的wheels=0,下面我们首先直接通过Car这个类来修改类变量的值:

# Change the class variable value

Car.wheels = 4

print(f"My car has {my_car.wheels} wheels")
print(f"My other car has {my_other_car.wheels} wheels")

Out:
My car has 4 wheels
My other car has 4 wheels

可以看到这样修改的话,Car类拥有的所有实例中的wheels值会被全部修改,如果我们通过my_other_car 来修改呢?

# Change the instance variable value for my_car

my_car.wheels = 5
print(f"My car has {my_car.wheels} wheels")
print(f"My other car has {my_other_car.wheels} wheels")

Out:
My car has 5 wheels
My other car has 4 wheels

现在大家可以发现区别了,仅仅是修改了my_car中wheels的值,对类本身不会造成影响

私有和公有属性

在Python中的所有属性都是public,可能有c++和java的同学觉得神奇,其实python最初规定了一种特殊的命名方式来区分public还是private,那就是下划线_

我还是拿一样的例子说明:

class Car:
    wheels = 0
    def __init__(self, color, model, year):
        self.color = color
        self.model = model
        self.year = year
        self._cupholders = 6

my_car = Car("yellow", "Beetle", "1969")
print(f"It was built in {my_car.year}")

Out:
It was built in 1969

这里Car类中的杯托 _cupholders就是“私有“属性,为什么我这里加上了引号,是因为Python只是名义上规定这种写法,但是在实际访问上没啥卵用,依然可以直接用._cupholders来访问:

my_car.year = 1966
print(f"It was built in {my_car.year}")
print(f"It has {my_car._cupholders} cupholders.")

Out:
It was built in 1966
It has 6 cupholders.

后来Python决定使用双下划线__来替换单下划线,这样可以最大程度避免“意外访问“,然而还是没有卵用,再来展示一下新方案:

class Car:
    wheels = 0
    def __init__(self, color, model, year):
        self.color = color
        self.model = model
        self.year = year
        self.__cupholders = 6

其实某种程度上,这回效果还是很明显的,如果我们还像刚才一样尝试调用my_car.cupholders 会报错:

my_car = Car("yellow", "Beetle", "1969")
print(f"It was built in {my_car.year}")
print(f"It has {my_car.__cupholders} cupholders.")


Out:
It was built in 1969

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-108-1efe56f0c054> in <module>
 1 my_car = Car("yellow", "Beetle", "1969")
 2 print(f"It was built in {my_car.year}")
----> 3  print(f"It has {my_car.__cupholders} cupholders.")

AttributeError: 'Car' object has no attribute '__cupholders'

这个错误很有意思,为什么会说cupholders这个变量不存在呢 ? 因为当Python看到__ 时,会自动在cupholders前面补上一个下划线_和所属类名,也就是说,这里我们尝试用my_car.__cupholders 来调用时,Python默认的正确写法是
my_car._Car__cupholders,现在再试一下:

print(f"It has {my_car._Car__cupholders} cupholders")
Out: It has 6 cupholders

看见没,依然没拦住。。。。
不过我个人认为这种规定公有私有变量的方式也是好处多多,这里就仁者见仁,智者见智了~

访问权限管理

就像刚刚提到的,Python所有的东西都是公有的,我们可以随意的新增,修改,甚至删除变量:

my_car = Car("yellow", "beetle", 1969)
print(f"My car was built in {my_car.year}")

my_car.year = 2003
print(f"It was built in {my_car.year}")

del my_car.year
print(f"It was built in {my_car.year}")

Out:
My car was built in 1969
It was built in 2003

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-110-46914b0bae82> in <module>
 6 
 7 del my_car.year
----> 8  print(f"It was built in {my_car.year}")

AttributeError: 'Car' object has no attribute 'year'

那我们如何才能控制属性的访问权限呢?Python给出的答案是装饰器 @property,这个类似于Java中的setter和getter,现在我们试试:

class Car:
    def __init__(self, color, model, year):
        self.color = color
        self.model = model
        self.year = year
        self._voltage = 12

    @property
    def voltage(self):
        return self._voltage

    @voltage.setter
    def voltage(self, volts):
        print("Warning: this can cause problems!")
        self._voltage = volts

    @voltage.deleter
    def voltage(self):
        print("Warning: the radio will stop working!")
        del self._voltage

我们新增了voltage(电压)这个属性,并用property来控制外部的访问权限,这里我们定义了三个方法,利用setter方法可以改变voltage的值,利用getter方法来访问,利用deleter方法实现删除,接下来让我们新建实例来看看propert是如何工作的:

my_car = Car("yellow", "beetle", 1969)
print(f"My car uses {my_car.voltage} volts")

my_car.voltage = 6
print(f"My car now uses {my_car.voltage} volts")

del my_car.voltage

Out:
My car uses 12 volts
Warning: this can cause problems!
My car now uses 6 volts
Warning: the radio will stop working!

可以发现,我们这里直接使用.voltage 而不是._voltage,这样就告诉python去使用property装饰的方法,我们可以通过使用@.setter and @.deleter 使属性变为read-only(只读),从而保护voltage不会被随意修改和删除

总结

今天主要总结了OOP编程中的类,对象,属性,公有私有属性,访问权限这些基础概念,下一篇文章会进一步深入,如果本文有哪些语言使用不当,希望大家可以指出,让我们一起进步!

我之前的一些文章已经放到了Github上,如果感兴趣的朋友可以去看看,链接如下:


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