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): //子类直接调用父类方法
        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 is called")
        Entity.__init__(self,'video') //显示调用父类属性
        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())

a= 3
def b(a):
    print(a)
    print(id(a))
    a = a+1
    print(id(a))
b(a)

l = 1
def multiply_2(l): 
    print(id(l))
    l=2
    #for index in range(0, len(l)): l[index] *= 2 
    print(id(l)) 
    return l

# print(id(l))  
multiply_2(l)
# print(l) 

int不可变，内存地址变；list可变，内存地址不变 ：新开辟内存空间，值不会变

抽象类，加上@abc.abstractmethod装饰器后严格控制子类必须实现这个方法

from abc import ABCMeta,abstractmethod

class Entity(metaclass=ABCMeta):

    @abstractmethod
    def get_title(self):
        pass

    @abstractmethod
    def set_title(self):
        pass

class Document(Entity):

    def get_title(self):
        return self.title

    def set_title(self,title):
        self.title = title

document = Document()
document.set_title("Harrie Potter")
print(document.get_title())

# 抽象类就是这么一种存在，它是一种自上而下的设计风范，你只需要用少量的代码描述清楚要做的事情，定义好接口，然后就可以交给不同开发人员去开发和对接。