type,元类,类,对象
简单描述
1.type是python内建元类,新建的元类需要继承type
2.元类用来创建类,类用来创建对象
类的生成方式
- 传统定义通过声明关键字class
#1.创建Person类
#2.继承object
#3.拥有属性eat
class Person(object):
def eat(self,food="rice"):
print("eat %s"%food)
p = Person()
p.eat(food="fruits")
- 通过type生成
注:该方法一般很少用,一般先由type生成元类,再有元类生成类,由类衍生出对象实例
def eat(self,food="rice"):
print("eat %s"%food)
People = type("People",(object,),{"eat":eat})
#People = type("People",(object,),dict(eat=eat))
p = People()
p.eat()
元类
一般来说元类均被命名为以MetaClass为后缀,元类是由type衍生出来,所以需继承type,元类的操作都在new方法中完成,new方法在init方法之前调用
- 示例一
#方式1,使用lambda匿名函数
class LanguageMetaClass(type):
def __new__(cls, name, bases, attrs):
attrs['speak'] = lambda self:print(self.language)
return type.__new__(cls,name,bases,attrs)
#方式2,使用预定义的函数
def speak(self):
print("speak %s"%self.language)
#创建元类LanguageMetaClass
class LanguageMetaClass(type):
def __new__(cls, name, bases, attrs):
attrs['speak'] = speak
#return type.__new__(cls,name,bases,attrs)
return super(LanguageMetaClass,cls).__new__(cls.name,bases,attrs)
#使用创建好的元类创建类(python2.x与3.x方式不一致)
#python2.x
#默认新式类由type()构造,当在类中定义了__metaclass__则由其构建,__metaclass__查找顺序:
#1.查找自身的__metaclass__属性
#2.如果有父类,查找父类的__metaclass__(通过父类.__class__属性获取)
#3.如果没有父类,查找模块中的__metaclass__
class People(object):
__metaclass__ = LanguageMetaClass
def __init__(self,language="chinese"):
self.language = language
#python3.x
#默认新式类由type()构造,可以通过指定metaclass关键字参数后,则由metaclass构造
class People(object,metaclass=LanguageMetaClass):
def __init__(self,language="chinese"):
self.language = language
p1 = People(language="English")
p1.speak() #speak English
p2 = People()
p2.speak() #speak chinese
- 示例二
#创建元类AddMetaClass
class AddMetaClass(type):
def __new__(cls,name,bases,attrs):
attrs['add'] = lambda self,value:self.append(value)
return type.__new__(cls,name,bases,attrs)
#python2.x
class MyList(list): #MyLIST在list的基础上多了个add方法
__metaclass__ = AddMetaClass
pass
#python3.x
class MyList(list,metaclass=AddMetaClass): #MyLIST在list的基础上多了个add方法
pass
list1 =MyList()
list1.add(1)
print(list1) #[1]
list1.add(2)
print(list1) #[1,2]