[Python-Dev] 'hasattr' is broken by design

[Yury Selivanov]

> On 2010-08-23, at 10:56 AM, Guido van Rossum wrote:
> On Mon, Aug 23, 2010 at 7:46 AM, Benjamin Peterson <benjamin at python.org> wrote:
>> 2010/8/23 Yury Selivanov <yselivanov at gmail.com>:
>>> 1) I propose to change 'hasattr' behaviour in Python 3, making it to swallow only AttributeError exceptions (exactly like 'getattr').  Probably, Python 3.2 release is our last chance.
>> 
>> I would be in support of that.
> 
> I am cautiously in favor. The existing behavior is definitely a
> mistake and a trap. But it has been depended on for almost 20 years
> now.
> 
> I recommend that you create a patch, apply it, run the *entire* stdlib
> test suite and see how much breaks. That will give you an idea of the
> damage to expect for 3rd party code.

Have done a little testing.

The patch to builtin 'hasattr' function is trivial, and can be found 
attached to this letter (with the corresponding unittest):

-        if (!PyErr_ExceptionMatches(PyExc_Exception))
+        if (!PyErr_ExceptionMatches(PyExc_AttributeError))

So, after applying it, hasattr swallows only AttributeError exceptions.


All tests from Python 3.2 test suite were good.

After that, I've applied the patch on Python 2.6 and tested SqlAlchemy,
Django and Twisted.

- SqlAlchemy has failed three unittests, but those tests were designed
specifically to handle 'hasattr' weird behaviour, so we can consider
the change has no impact on SqlAlchemy.

- Twisted - failed 3 tests out of ~3200, but it fails them on the same
machine on an unpatched Python too, and they seem unrelated.

- Django - all tests passed.


I tested our internal company framework (~100K LOC) and, again, all
tests passed.


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