Python基础:一起来面向对象 (一)

类，一群有着相同属性和函数的对象的集合

　　如果你不满足于只做一个+CRUD“码农”，而是想成为一个优秀的工程师，那就一定要积极锻炼直觉思考和快速类比的能力，其是在找不到bug的时候

　　

　　类的示例:

class Document():
    def __init__(self, title, author, context):
        print('init function called')
        self.title = title
        self.author = author
        self.__context = context # __ 开头的属性是私有属性

    def get_context_length(self):
        return len(self.__context)

    def intercept_context(self, length):
        self.__context = self.__context[:length]

harry_potter_book = Document('Harry Potter', 'J. K. Rowling', '... Forever Do not believe any thing is capable of thinking independently ...')

print(harry_potter_book.title)
print(harry_potter_book.author)
print(harry_potter_book.get_context_length())

harry_potter_book.intercept_context(10)

print(harry_potter_book.get_context_length())

print(harry_potter_book.__context)

########## 输出 ##########

init function called
Harry Potter
J. K. Rowling
77
10

---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-5-b4d048d75003> in <module>()
     22 print(harry_potter_book.get_context_length())
     23 
---> 24 print(harry_potter_book.__context)

AttributeError: 'Document' object has no attribute '__context'

三问对象

　　1.如何在一个类中定义一些常量，每个对象都可以方便访问这些常量而不用重新构造？

　　答: 一般写成大字，与函数并列声明并赋值，注意是可以修改的。

　　2.如果一个函数不涉及到访问修改这个类的属性，而放到类外面有点不恰当，怎么做才能更优雅呢？

　　答: classmethod装饰器声明函数为类函数，最常用的功能用于定义不同的init函数，如create_empty_book函数创建的对象 context一定为'nothing',这比直接构造要清晰一些.

　　     staticmethod装饰器声明函数为静态函数，可以用来做一些简单独立的任务

class Document():
    
    WELCOME_STR = 'Welcome! The context for this book is {}.'
    
    def __init__(self, title, author, context):
        print('init function called')
        self.title = title
        self.author = author
        self.__context = context
    
    # 类函数
    @classmethod
    def create_empty_book(cls, title, author):
        return cls(title=title, author=author, context='nothing')
    
    # 成员函数
    def get_context_length(self):
        return len(self.__context)
    
    # 静态函数
    @staticmethod
    def get_welcome(context):
        return Document.WELCOME_STR.format(context)


empty_book = Document.create_empty_book('What Every Man Thinks About Apart from Sex', 'Professor Sheridan Simove')


print(empty_book.get_context_length())
print(empty_book.get_welcome('indeed nothing'))

######### 输出 ##########

init function called
7
Welcome! The context for this book is indeed nothing.

　　3.既然类是一群相似的对象的集合，那么可不可以是一群相似的类的集合呢？

• 继承类在生成对象时不会自动调用父类的构造函数，必须在init()函数中显式调用父类中的构造函数 Entity.__init__(self, 'document')
• 当子类对象调用 get_context_length 函数时，如果没有实现此函数，会抛出get_context_length not implemented异常，这就使子类必须实现此函数。
• 父类中的print_title 函数体现了继承的优势，减少重复代码.

class Entity():
    def __init__(self, object_type):
        print('parent class init called')
        self.object_type = object_type
    
    def get_context_length(self):
        raise Exception('get_context_length not implemented')
    
    def print_title(self):
        print(self.title)

class Document(Entity):
    def __init__(self, title, author, context):
        print('Document class init called')
        Entity.__init__(self, 'document')
        self.title = title
        self.author = author
        self.__context = context
    
    def get_context_length(self):
        return len(self.__context)
    
class Video(Entity):
    def __init__(self, title, author, video_length):
        print('Video class init called')
        Entity.__init__(self, 'video')
        self.title = title
        self.author = author
        self.__video_length = video_length
    
    def get_context_length(self):
        return self.__video_length

harry_potter_book = Document('Harry Potter(Book)', 'J. K. Rowling', '... Forever Do not believe any thing is capable of thinking independently ...')
harry_potter_movie = Video('Harry Potter(Movie)', 'J. K. Rowling', 120)

print(harry_potter_book.object_type)
print(harry_potter_movie.object_type)

harry_potter_book.print_title()
harry_potter_movie.print_title()

print(harry_potter_book.get_context_length())
print(harry_potter_movie.get_context_length())

########## 输出 ##########

# Document class init called
# parent class init called
# Video class init called
# parent class init called
# document
# video
# Harry Potter(Book)
# Harry Potter(Movie)
# 77
# 120

抽像类的使用

　　抽象类是为父类而生的，不能对象化。如果有抽象函数也必须在子类中重写才能使用。

from abc import ABCMeta, abstractmethod

class Entity(metaclass=ABCMeta):
    @abstractmethod
    def get_title(self):
        pass

    @abstractmethod
    def set_title(self, title):
        pass

class Document(Entity):
    def get_title(self):
        return self.title
    
    def set_title(self, title):
        self.title = title

document = Document()
document.set_title('Harry Potter')
print(document.get_title())

entity = Entity()

######### 输出 ##########
Harry Potter

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-7-266b2aa47bad> in <module>()
     21 print(document.get_title())
     22 
---> 23 entity = Entity()
     24 entity.set_title('Test')

TypeError: Can't instantiate abstract class Entity with abstract methods get_title, set_title

参考:

　　极客时间《Python核心技术与实战》