Python之描述器

#描述器A调用并赋值给了类B的属性，当调用类B或者实例b时，去类A执行__get__()函数，类调用instance返回none，实例调用返回实例
#执行顺序和类，实例字典无关
class A:
    def __init__(self):
        print(2,‘A init‘)
    def __set__(self, instance, value):
        print(3,self,instance,value)
    def __get__(self, instance, owner): #return值，将会影响b.x or B.x的调用属性
        print(4,self,instance,owner)

class B:
    x = A()
    def __init__(self):
        print(1,‘B init‘)

b = B()  #output 2->1
b.x   #output 4 <__main__.A object at 0x047B09D0> <__main__.B object at 0x047BB350> <class ‘__main__.B‘>
B.x   #output 4 <__main__.A object at 0x047B09D0> None <class ‘__main__.B‘>

此时访问b.x.a1  B.x.a1都会报错 AttributeError:Nonetype
问题出在__get__的返回值，修改为 return self 返回A的实例就具备了a1属性 返回正常

class B:
    x = A()
    def __init__(self):
        self.b1 = A()
        print(1,‘B init‘)

b = B()  #output 2->1
print(b.b1) #output <__main__.A object at 0x03000990> 没有触发__get__的打印
从运行结果可以看出，只有类属性是类的实例才行

class A:
    def __init__(self):
        self.a1 = ‘a1‘
        print(2,‘A init‘)

    def __get__(self, instance, owner):
        print(4,self,instance,owner)
        return self

class B:
    x = A()
    def __init__(self):
        self.x = ‘b1‘   #如果描述器定义了__set__,此时b1就是value
        print(1,‘B init‘)

b = B()  #output 2->1
print(B.x) #output 4  <__main__.A object at 0x04EEB350> None <class ‘__main__.B‘> ;;;; return <__main__.A object at 0x02E8B350>
print(B.x.a1) #output 4  <__main__.A object at 0x02E8B350> None <class ‘__main__.B‘> ;;;;return a1
print(b.x)  #return b1 访问到了实例的属性，而不是描述器
print(b.x.a1) #AttributeError ‘str object has no Attribute
在非数据描述器中，owner实例的属性 会被实例调用，而不是访问__get__描述器

#添加了set方法 对比上个代码，；数据描述器
class A:
    def __init__(self):
        self.a1 = ‘a1‘
        print(2,‘A init‘)
    def __set__(self, instance, value):  #当定义了set魔术方法后，B实例定义的实例属性self.x = ‘b1 不会在写进实例字典，而是调用set方法
        print(3,self,instance,value)
        # instance.__dict__[‘x‘]=value
    def __get__(self, instance, owner): #return值，将会影响b.x or B.x的调用属性
        print(4,self,instance,owner)
        # return instance.__dict__[‘x‘]
        return self
class B:
    x = A()
    def __init__(self):
        print(1,‘B init‘)
        print("+++++++++++++++++++++++")
        self.x = ‘b1‘
        print("+++++++++++++++++++++++")

b = B()  #output 2->1->+ ->3-> + ；；实例化时候，self.x = ‘b1‘调用了set方法
print(b.x.a1)  #return a1 直接调用get
b.x = 100       #return a1 直接调用set
print(b.__dict__)  #实例字典为空
print(B.__dict__)

class A:
    def __init__(self):
        self.a1 = ‘a1‘
        print(2,‘A init‘)
    def __set__(self, instance, value):
        print(3,self,instance,value)
        self.data = value
        print(self.data)
    def __get__(self, instance, owner):
        print(4,self,instance,owner)
        return self
class B:
    x = A()
    def __init__(self):
        print(1,‘B init‘)
        self.x = ‘b.x‘
        self.y = ‘b.y‘
        self.z = ‘b.z‘


b = B()  #output 2->1->+ ->3-> + ；；实例化时候，self.x = ‘b1‘调用了set方法
print(b.y) #return b.y
print(b.x)
print(B.__dict__)
print(b.__dict__)#output {‘y‘: ‘b.y‘, ‘z‘: ‘b.z‘}  ;;;self.x 这里的x是x = A()所以调用set方法，而self.y self.z追加到字典

#StaticMethod
from functools import partial
class StaticMethod:
    def __init__(self,x):
        self.x = x 
    def __get__(self, instance, owner):
        return self.x

class ClassMethod:
    def __init__(self,x):
        self.x = x

    def __get__(self, instance, owner):
        return partial(self.x,owner)

class A:
    @StaticMethod 
    def a():
        print("a")
    @ClassMethod  #b = classmethod(b);
    def b(cls):
        print("b")

A().b()
A.b()
A.a()
A().a()