Python的hasattr有时返回不正确的结果

问题描述

为什么hasattr说实例没有foo属性?

>>> class A(object):
...     @property
...     def foo(self):
...         ErrorErrorError
...
>>> a = A()
>>> hasattr(a, 'foo')
False

我期望:

>>> hasattr(a, 'foo')
NameError: name 'ErrorErrorError' is not defined`

解决方案

hasattr 相当幼稚，它只是尝试访问该属性并查看它是否引发异常.

不幸的是，这意味着属性中任何未处理的异常都将被吞噬，并且该代码中的错误可能会丢失.为了增加侮辱性的伤害，当hasattr吞噬异常时，它还会返回错误的答案(此处属性a.foo 确实存在，因此结果应该返回True)./p>

在python3.2 +中，行为已得到纠正:

此修复程序是此处，但很遗憾，该更改并未向后移植./p>

如果python2行为给您带来麻烦，请考虑避免使用hasattr；相反，您可以在 getattr 周围使用try/except，仅捕获AttributeError异常，并让其他任何异常都无法处理.

Why does hasattr say that the instance doesn't have a foo attribute?

>>> class A(object):
...     @property
...     def foo(self):
...         ErrorErrorError
...
>>> a = A()
>>> hasattr(a, 'foo')
False

I expected:

>>> hasattr(a, 'foo')
NameError: name 'ErrorErrorError' is not defined`

解决方案

The python2 implementation of hasattr is fairly naive, it just tries to access that attribute and see whether it raises an exception or not.

Unfortunately, this means that any unhandled exceptions inside properties will get swallowed, and errors in that code can get lost. To add insult to injury, when hasattr eats the exception, it will also return an incorrect answer (here the attribute a.foo does exist, so the result should have returned True if anything).

In python3.2+, the behaviour has been corrected:

The fix is here, but unfortunately that change didn't backport.

If the python2 behaviour causes trouble for you, consider to avoid using hasattr; instead you can use a try/except around getattr, catching only the AttributeError exception and letting any others raise unhandled.

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