Python 面向对象编程OOP (一) 类，对象，属性，访问权限

摘要：面向对象编程之旅编程是什么大家好，作为小白，最近学习了很多编程的知识，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。。。继承也允许把一个派生类的对象作为一个基类对象对待。

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：

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）

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

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"]

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

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)
 in 
 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~

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)
 in 
 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

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：

Python 精品练习题100道

Python 实用技巧汇总

Python Pandas教程