大家好，作為小白，最近學(xué)習(xí)了很多Python OOP編程的知識，因為腦容量有限，特此一一按照學(xué)習(xí)順序記錄下來，如果哪里有錯誤，還請大神盡快指出，以免誤導(dǎo)他人。。。

首先讓我們簡單了解一下何為面向?qū)ο缶幊蹋?/p>

把一組數(shù)據(jù)結(jié)構(gòu)和處理它們的方法組成對象（object），把相同行為的對象歸納為類（class），通過類的封裝（encapsulation）隱藏內(nèi)部細(xì)節(jié)，通過繼承（inheritance）實現(xiàn)類的特化（specialization）和泛化（generalization），通過多態(tài)（polymorphism）實現(xiàn)基于對象類型的動態(tài)分派。

這樣一說貌似有些復(fù)雜，簡單來看的話可以參考下面的解釋：

下面讓我們簡單定義一個汽車類：

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

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

這里我們創(chuàng)建了一個汽車類Car，它有三個公共屬性，分別是color（顏色），model（型號），year（生產(chǎn)年份）

現(xiàn)在讓我們新建一個對象my_car:

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

查看一下my_car的屬性

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

我們想要給my_car添加一個新屬性wheels

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

使用dir(my_car)可以讓我們確認(rèn)一下屬性是否存在：

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",     <====已經(jīng)添加成功啦
 "year"]

在Python中，我們在類外聲明一個類變量，下面讓我們修改一下Car類：

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

這樣的話，我們在調(diào)用wheels這個變量時，可以通過實例，或者直接調(diào)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

這里需要注意一下，如果想要通過my_car.wheels =xxx來修改wheels的值，不會真正修改類變量wheels的值，我們來看一個具體的例子：

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

我們首先創(chuàng)建兩個實例my_car 和my_other_car ，默認(rèn)的wheels=0，下面我們首先直接通過Car這個類來修改類變量的值：

# 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

可以看到這樣修改的話，Car類擁有的所有實例中的wheels值會被全部修改，如果我們通過my_other_car 來修改呢？

# 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

現(xiàn)在大家可以發(fā)現(xiàn)區(qū)別了，僅僅是修改了my_car中wheels的值，對類本身不會造成影響

在Python中的所有屬性都是public，可能有c++和java的同學(xué)覺得神奇，其實python最初規(guī)定了一種特殊的命名方式來區(qū)分public還是private，那就是下劃線_

我還是拿一樣的例子說明：

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

這里Car類中的杯托 _cupholders就是“私有“屬性，為什么我這里加上了引號，是因為Python只是名義上規(guī)定這種寫法，但是在實際訪問上沒啥卵用，依然可以直接用._cupholders來訪問：

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.

后來Python決定使用雙下劃線__來替換單下劃線，這樣可以最大程度避免“意外訪問“，然而還是沒有卵用，再來展示一下新方案：

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

其實某種程度上，這回效果還是很明顯的，如果我們還像剛才一樣嘗試調(diào)用my_car.cupholders 會報錯：

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"

這個錯誤很有意思，為什么會說cupholders這個變量不存在呢 ? 因為當(dāng)Python看到__ 時，會自動在cupholders前面補(bǔ)上一個下劃線_和所屬類名，也就是說，這里我們嘗試用my_car.__cupholders 來調(diào)用時，Python默認(rèn)的正確寫法是
my_car._Car__cupholders，現(xiàn)在再試一下：

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

看見沒，依然沒攔住。。。。
不過我個人認(rèn)為這種規(guī)定公有私有變量的方式也是好處多多，這里就仁者見仁，智者見智了~

就像剛剛提到的，Python所有的東西都是公有的，我們可以隨意的新增，修改，甚至刪除變量：

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"

那我們?nèi)绾尾拍芸刂茖傩缘脑L問權(quán)限呢？Python給出的答案是裝飾器 @property，這個類似于Java中的setter和getter，現(xiàn)在我們試試：

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

我們新增了voltage（電壓）這個屬性，并用property來控制外部的訪問權(quán)限，這里我們定義了三個方法，利用setter方法可以改變voltage的值，利用getter方法來訪問，利用deleter方法實現(xiàn)刪除，接下來讓我們新建實例來看看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!

可以發(fā)現(xiàn)，我們這里直接使用.voltage 而不是._voltage，這樣就告訴python去使用property裝飾的方法，我們可以通過使用@.setter and @.deleter 使屬性變?yōu)閞ead-only（只讀），從而保護(hù)voltage不會被隨意修改和刪除

今天主要總結(jié)了OOP編程中的類，對象，屬性，公有私有屬性，訪問權(quán)限這些基礎(chǔ)概念，下一篇文章會進(jìn)一步深入，如果本文有哪些語言使用不當(dāng)，希望大家可以指出，讓我們一起進(jìn)步!

我之前的一些文章已經(jīng)放到了Github上，如果感興趣的朋友可以去看看，鏈接如下：

Python 精品練習(xí)題100道

Python 實用技巧匯總

Python Pandas教程