http://www.cnblogs.com/linhaifeng/articles/6204014.html#_label12
描述符是什么:描述符本质就是一个新式类,在这个新式类中,至少实现了__get__(),__set__(),__delete__()中的一个,这也被称为描述符协议
__get__():调用一个属性时,触发
__set__():为一个属性赋值时,触发
__delete__():采用del删除属性时,触发
定义一个描述符
class Foo: #在python3中Foo是新式类,它实现了三种方法,这个类就被称作一个描述符 def __get__(self, instance, owner): pass def __set__(self, instance, value): pass def __delete__(self, instance): pass
描述符是干什么的:描述符的作用是用来代理另外一个类的属性的(必须把描述符定义成这个类的类属性,不能定义到构造函数中)
#描述符Str class Str: def __get__(self, instance, owner): print('Str调用') def __set__(self, instance, value): print('Str设置...') def __delete__(self, instance): print('Str删除...') #描述符Int class Int: def __get__(self, instance, owner): print('Int调用') def __set__(self, instance, value): print('Int设置...') def __delete__(self, instance): print('Int删除...') class People: name=Str() age=Int() def __init__(self,name,age): #name被Str类代理,age被Int类代理, self.name=name self.age=age #何地?:定义成另外一个类的类属性 #何时?:且看下列演示 p1=People('alex',18) ''' Str设置... Int设置... ''' # #描述符Str的使用 p1.name # Str调用 p1.name='zhangsan' # Str设置... del p1.name # Str删除... # # #描述符Int的使用 p1.age # Int调用 p1.age=18 # Int设置... del p1.age # Int删除... # #我们来瞅瞅到底发生了什么 print(p1.__dict__) # {} print(People.__dict__) # {'__weakref__': <attribute '__weakref__' of 'People' objects>, # '__dict__': <attribute '__dict__' of 'People' objects>, '__doc__': None, # '__init__': <function People.__init__ at 0x0000013E94AC5E18>, '__module__': '__main__', # 'name': <__main__.Str object at 0x0000013E94AD81D0>, 'age': <__main__.Int object at 0x0000013E94AD8198>} # #补充 print(type(p1) == People) #type(obj)其实是查看obj是由哪个类实例化来的 True print(type(p1).__dict__ == People.__dict__) # True
#描述符Str class Str: def __get__(self, instance, owner): print('Str调用') def __set__(self, instance, value): print('Str设置...') instance.__dict__['name'] = value def __delete__(self, instance): print('Str删除...') #描述符Int class Int: def __get__(self, instance, owner): print('Int调用') def __set__(self, instance, value): print('Int设置...') instance.__dict__['age'] = value def __delete__(self, instance): print('Int删除...') class People: name=Str() age=Int() def __init__(self,name,age): #name被Str类代理,age被Int类代理, self.name=name self.age=age p1=People('alex',18) print(p1.__dict__) # {'name': 'alex', 'age': 18}
描述符分两种
一 数据描述符:至少实现了__get__()和__set__()
class Foo: def __set__(self, instance, value): print('set') def __get__(self, instance, owner): print('get')
非数据描述符:没有实现__set__()
class Foo: def __get__(self, instance, owner): print('get')
注意事项:
一 描述符本身应该定义成新式类,被代理的类也应该是新式类
二 必须把描述符定义成这个类的类属性,不能为定义到构造函数中
三 要严格遵循该优先级,优先级由高到底分别是
1.类属性
2.数据描述符
3.实例属性
4.非数据描述符
5.找不到的属性触发__getattr__()
#描述符Str class Str: def __init__(self,key): self.key = key def __get__(self, instance, owner): print('Str调用') return instance.__dict__[self.key] def __set__(self, instance, value): print('Str设置...') if type(value) is not str: raise TypeError("类型错误") instance.__dict__[self.key] = value def __delete__(self, instance): print('Str删除...') instance.__dict__.pop(self.key) #描述符Int class Int: def __init__(self,key): self.key = key def __get__(self, instance, owner): print('Int调用') return instance.__dict__[self.key] def __set__(self, instance, value): print('Int设置...') if type(value) is not int: raise TypeError("请输入int型") instance.__dict__['age'] = value def __delete__(self, instance): print('Int删除...') instance.__dict__.pop(self.key) class People: name=Str("name") age=Int("age") def __init__(self,name,age): #name被Str类代理,age被Int类代理, self.name=name self.age=age p1=People("zhangsan",18) print(p1.__dict__) # {'age': 18, 'name': 'zhangsan'} print(p1.age) del p1.age print(p1.__dict__) # {'age': 18}
上面代码改进:
class Typed: def __init__(self,key, expected_type): self.key = key self.expected_type = expected_type def __get__(self, instance, owner): return instance.__dict__[self.key] def __set__(self, instance, value): if not isinstance(value, self.expected_type): raise TypeError("%s is not %s" %(value, self.expected_type)) instance.__dict__[self.key] = value def __delete__(self, instance): instance.__dict__.pop(self.key) class People: name=Typed("name", str) age=Typed("age", int) def __init__(self,name,age): #name被Str类代理,age被Int类代理, self.name=name self.age=age #p1=People(99,18) # TypeError: 99 is not <class 'str'> p1=People("zhangsan",'18') # TypeError: 18 is not <class 'int'>