Question

在此示例代码中，我想确定x是TestProperty的实例：

class TestProperty(object):
    def __init__(self, name):
        self._name = name

    def __get__(self, instance, cls):
        return getattr(instance, self._name)

    def __set_(self, instance, value):
        setattr(instance, self._name, value)

class Test(object):
    x = TestProperty("x")

print isinstance(Test.x, TestProperty)

但是，我得到以下例外：

Traceback (most recent call last):
  File "/home/zenoss/testproperties.py", line 14, in <module>
    print isinstance(Test.x, TestProperty)
  File "/home/zenoss/testproperties.py", line 6, in __get__
    return getattr(instance, self._name)
AttributeError: 'NoneType' object has no attribute 'x'

无论如何，当它是一个描述符时，是否有一个属性是一个类的实例？

Answer 1

使用当前__get__，Test.x会导致AttributeError，因为当使用类instance访问描述符的代码传递None时; （=＆gt; getattr(None, 'x') =＆gt; None.x）

你应该修改__get__来处理这种情况：

>>> class TestProperty(object):
...     def __init__(self, name):
...         self._name = name
...     def __get__(self, instance, cls):
...         if instance is None:  # To handle access through class, not instance
...             return self       # returns the TestProperty instance itself.
...         return getattr(instance, self._name)
...     def __set_(self, instance, value):
...         setattr(instance, self._name, value)
... 
>>> class Test(object):
...     x = TestProperty("x")
... 
>>> isinstance(Test.x, TestProperty)
True

正如您所知，BTW，x = TestProperty("x")，通过实例访问x属性会导致另一个异常，因为它会调用__get__（ - ＆gt; getattr（.. ） - ＆gt; __get__ - ＆gt; getattr（..） - ＆gt; ...）递归直到堆栈溢出。

Answer 2

实现属性的最佳方法是使用@property装饰器：

class TestProperty(object):

    def __init__(self, name):
        self._name = name

    @property
    def name(self):
        """Getter for '_name'."""
        return self._name

    @name.setter
    def name(self, value):
        """Setter for '_name'."""
        self._name = value

class Test(object):
    x = TestProperty("x")

print(isinstance(Test.x, TestProperty))

运行时返回True。请参阅https://docs.python.org/3/library/functions.html#property上的@property文档。

如何确定描述符的类？

2 个答案: