Python描述符

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描述符

定义

  1. 一个描述符是一个有“绑定行为”的对象属性(object attribute)，它的访问控制会被描述器协议方法重写。
  2. 任何定义了 __get__, __set__ 或者 __delete__ 任一方法的类称为描述符类，其实例对象便是一个描述符，这些方法称为描述符协议。
    • 同时定义了 __get__ 和 __set__ 的描述符称为 数据描述符(data descriptor)；仅定义了 __get__ 的称为 非数据描述符(non-data descriptor) 。
    • 两者区别在于：如果 obj.__dict__ 中有与描述符同名的属性，若描述符是数据描述符，则优先调用描述符，若是非数据描述符，则优先使用 obj.__dict__ 中属性。
  3. 当对一个实例属性进行访问时，Python 1、类属性，2、数据描述符，3、实例属性，4、非数据描述符，5、__getattr__()顺序进行查找，如果查找到目标属性并发现是一个描述符，Python 会调用描述符协议来改变默认的控制行为。
  4. 描述符是 @property，@classmethod，@staticmethod 和 super 的底层实现机制。

描述符方法

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__get__(self, instance, owner)
__set__(self, instance, value)
__delete__(self, instance)

1. 使用类方法创建描述符

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class Descriptor(object):
    def __init__(self):
        self._name = ''

    def __get__(self, instance, owner):
        print("Getting: %s" % self._name)
        return self._name

    def __set__(self, instance, name):
        print("Setting: %s" % name)
        self._name = name.title()

    def __delete__(self, instance):
        print("Deleting: %s" %self._name)
        del self._name
 
class Person(object):
    name = Descriptor()

2. 使用属性类型创建描述符

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class Person(object):
    def __init__(self):
        self._name = ''

    def fget(self):
        print("Getting: %s" % self._name)
        return self._name

    def fset(self, value):
        print("Setting: %s" % value)
        self._name = value.title()

    def fdel(self):
        print("Deleting: %s" %self._name)
        del self._name

    # property() 的语法是 property(fget=None, fset=None, fdel=None, doc=None)
    name = property(fget, fset, fdel, "I'm the property.")

3. 使用属性修饰符（proprty）创建描述符

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class Person(object):
    def __init__(self):
        self._name = ''

    @property
    def name(self):
        print "Getting: %s" % self._name
        return self._name

    @name.setter
    def name(self, value):
        print "Setting: %s" % value
        self._name = value.title()

    @name.deleter
    def name(self):
        print ">Deleting: %s" % self._name
        del self._name

staticMethod 原理

@staticmethod : when this method is called, we don’t pass an instance of the class to it (as we normally do with methods). This means you can put a function inside a class but you can’t access the instance of that class (this is useful when your method does not use the instance).

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class StaticMethod(object):
    def __init__(self, f):
        self.f = f
    
    def __get__(self, obj, objtype=None):
        return self.f


class E(object):
    @StaticMethod
    def f( x):
        return x
        
print(E.f('staticMethod Test'))
ee = E()
print(ee.f('ee static'))

classmethod 原理

A class method receives the class as implicit first argument, just like an instance method receives the instance. To declare a class method, use this idiom.
A class method can be called either on the class (such as C.f()) or on an instance (such as C().f()). The instance is ignored except for its class. If a class method is called for a derived class, the derived class object is passed as the implied first argument.

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class ClassMethod(object):
    def __init__(self, f):
        self.f = f
    
    def __get__(self, obj, owner=None):
        if owner is None:
            owner = type(obj)
        
        def newfunc(*args):
            return self.f(owner, *args)
        
        return newfunc

class E(object):
    name = 'name'
    @ClassMethod
    def f(cls,x):
        return x + cls.name

print(E.f('classMethod Test'))

参考:
Python 描述符简介
Python 描述符(Descriptor) 附实例
【案例讲解】Python为什么要使用描述符？
python描述符(descriptor)、属性(property)、函数（类）装饰器(decorator )原理实例详解
python装饰器、描述符模拟源码实现