Python基础:一起来面向对象 (一)
2024-08-26 07:52:43
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)
#
#
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》
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