property的AttributeError的传播问题

前言

去年我写过一篇你用对 hasattr 了嘛？介绍过被 property 装饰的方法内部抛错会引起 hasattr 的结果为 False。

今天又遇到了一个 AttributeError 向上传播的问题 (Python 2)，一起来看看

问题

先上代码:

In : class T(object):
...:     @property
...:     def name(self):
...:         print(self.missing_attribute)
...:         return 42
...:     def __getattr__(self, name):
...:         raise AttributeError(name)
...:

In : t = T()

In : t.name
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-6-afc812d51b9a> in <module>()
----> 1 t.name

<ipython-input-4-a0ba2d3a6446> in __getattr__(self, name)
      5         return 42
      6     def __getattr__(self, name):
----> 7         raise AttributeError(name)
      8

AttributeError: name

In : [k for k in dir(t) if not k.startswith('_')]
Out: ['name']

这个例子需要细品，明明有 name 这个属性却抛出了 AttributeError。分析一下:

通过 __getattr__ 定制属性查找的逻辑，当用户试图获取一个不存在的属性时就会抛 AttributeError 错误
获取 name 属性的逻辑中包含 print (self.missing_attribute) 这句，由于没有这个属性会执行到 __getattr__ 然后抛 AttributeError

一看到这个违背常理的错误，我就想起来 hasattr 的那个问题，觉得应该是类似的原因。网上一搜果然之前有人已经给 CPython 提过 issue 了，具体可以看延伸阅读链接。总结一下，这个问题的出现需要 2 个点都满足：

实现了 __getattr__
使用 property 装饰器的方法的逻辑中有未预期的属性调用

相当于方法中的 AttributeError 没有被处理，传播到__getattr__了。再看一个例子:

In : class T(object):
...:     @property
...:     def name(self):
...:         raise AttributeError('This message will not be displayed!')
...:         return 'Hello'
...:     def __getattr__(self, name):
...:         return 0
...:

In : t = T()

In : t.name
Out: 0

一旦 name 方法中由于各种原因会抛 AttributeError 错误 (别的错误不行)，就会走__getattr__里面的逻辑。

怎么解决？

这不是一个 BUG，但是代码编写者不能规避这类问题，怎么办呢？最简单的办法是使用__getattribute__替代__getattr__，演示一下:

In : class T(object):
...:     @property
...:     def name(self):
...:         raise AttributeError('This message will not be displayed!')
...:         return 'Hello'
...:
...:     def __getattribute__(self, name):
...:         try:
...:             return super(T, self).__getattribute__(name)
...:         except AttributeError as e:
...:             raise e
...:

In : t = T()

In : t.name
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-28-afc812d51b9a> in <module>()
----> 1 t.name

<ipython-input-26-a27dd1330f44> in __getattribute__(self, name)
      9             return super(T, self).__getattribute__(name)
     10         except AttributeError as e:
---> 11             raise e
     12

AttributeError: This message will not be displayed!

In : class T(object):
...:     @property
...:     def name(self):
...:         print(self.missing_attribute)
...:         return 42
...:     def __getattribute__(self, name):
...:         try:
...:             return super(T, self).__getattribute__(name)
...:         except AttributeError as e:
...:             raise e
...:

In : t = T()

In : t.name
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-31-afc812d51b9a> in <module>()
----> 1 t.name

<ipython-input-29-da47b1a1e093> in __getattribute__(self, name)
      8             return super(T, self).__getattribute__(name)
      9         except AttributeError as e:
---> 10             raise e
     11

AttributeError: 'T' object has no attribute 'missing_attribute'

就是这样~