Python 的描述器Descriptors

描述器的表現
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用到3個魔術方法： __get__()\__set__()\__delete__()

方法簽名：
object.__get__(self, instance, owner)
object.__set__(self, instance, value)
object.__delete__(self, instance)

self 指當前實例，調用者
instance是owner的實例
owner是屬性的所屬的類

描述器定義
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Python中，一個類實現了__get__()\__set__()\__delete__()中的任一種方法，就是描述器。

非數據描述符：僅實現__get__
數據描述符：實現__get__ \ __set__

如果一個類的類屬性設置為描述器，它被稱為owner屬主

屬性的訪問順序
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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’ #增加實例屬性

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 #調用數據數據描述符的__set__方法，或非數據描述器的實例覆蓋
B.x = 600 #賦值即定義，覆蓋類屬性
print(b.x)
print(b.__dict__)
print(B.__dict__)

屬性查找順序：
實例的__dict__優先于非數據描述符
數據描述符優先于實例的__dict__ (屬性查找變成了描述器的__set__)

Python中的描述器應用
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staticmethod() 和 classmethod()是非數據描述器。實例可以重新定義和覆蓋方法。

property()是數據描述符。實例不能覆蓋屬性的行為。

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)