描述器的表现
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用到3个魔术方法: __get__()\__set__()\__delete__()
方法签名:
object.__get__(self, instance, owner)
object.__set__(self, instance, value)
object.__delete__(self, instance)
self 指当前实例,调用者
instance是owner的实例
owner是属性的所属的类
描述器定义
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Python中,一个类实现了__get__()\__set__()\__delete__()中的任一种方法,就是描述器。
非数据描述符:仅实现__get__
数据描述符:实现__get__ \ __set__
如果一个类的类属性设置为描述器,它被称为owner属主
属性的访问顺序
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class A:
def __init__(self):
self.a1 = ‘a1’
print(‘A init’)
def __get__(self, instance, owner):
print(“A.__get__{} {} {}”.format(self, instance, owner))
return self
def __set__(self, instance, value):
print(“A.__set__{} {} {}”.format(self , instance , value))
self.data = value
def __delete__(self, instance):
print(“A.__delete__{} {}”.format(self , instance))
del self
class B:
x = A()
def __init__(self):
print(‘B init’)
self.y = ‘y1’
self.x = ‘b.x’ #增加实例属性
print(‘-‘*20)
print(B.x)
print(B.x.a1)
print(‘=’*20)
b = B()
print(b.x)
print(b.x.data)
print(b.__dict__)
b.x.a1 = 200
print(b.x.__dict__)
print(b.__dict__)
del b.x
print(b.x)
print(b.x.a1)
print(b.__dict__)
print(‘+’*20)
b.x = 300 #调用数据数据描述符的__set__方法,或非数据描述器的实例覆盖
B.x = 600 #赋值即定义,覆盖类属性
print(b.x)
print(b.__dict__)
print(B.__dict__)
属性查找顺序:
实例的__dict__优先于非数据描述符
数据描述符优先于实例的__dict__ (属性查找变成了描述器的__set__)
Python中的描述器应用
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staticmethod() 和 classmethod()是非数据描述器。实例可以重新定义和覆盖方法。
property()是数据描述符。实例不能覆盖属性的行为。
class staticmethod:
def __init__(self, fn):
self._fn = fn
print(‘1 init’)
def __get__(self, instance, owner):
print(“1.__get__{} {} {}”.format(self, instance, owner))
return self._fn
class classmethod:
def __init__(self , fn):
self._fn = fn
print(‘2 init’)
def __get__(self, instance , owner):
from functools import partial
print(“2.__get__{} {} {}”.format(self , instance , owner))
return partial(self._fn, owner)
class property:
def __init__(self, fn):
self._fn = fn
self.setfn = None
print(‘3 init’)
def __get__(self, instance, owner):
print(“3.__get__{} {} {}”.format(self , instance , owner))
return self._fn(instance)
def __set__(self, instance, value):
print(“3.__set__{} {} {}”.format(self , instance , value))
self.setfn(instance,value)
# def getter(self):
# return self.__get__
def setter(self,fn):
self.setfn = fn
return self
class B:
@staticmethod # add = staticmethod(add) -> add
def add(x, y):
return x + y
@classmethod # add = staticmethod(add) -> add
def add(cls, x, y):
return x + y
@property # x = property(x) -> x
def x(self):
return self._x
@x.setter # x = x.setter(x) -> x
def x(self, value):
self._x = value
def __init__(self):
self._x = 6
b = B()
# print(b.add(3, 5))
# print(b.__dict__)
# print(B.__dict__)
print(b.x)
b.x = 10
print(b.x)
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