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

[Michael Foord]

  On 24/08/2010 08:40, Michael Foord wrote:
> On 24/08/2010 01:25, Nick Coghlan wrote:
>> On Tue, Aug 24, 2010 at 8:15 AM, Nick Coghlan<ncoghlan at gmail.com> wrote:
>>> Now, it may be worth considering an addition to the inspect module
>>> that was basically:
>>>
>>> def getattr_static(obj, attr):
>>> """Retrieve attributes without triggering dynamic lookup via the
>>> descriptor protocol,
>>> __getattr__ or __getattribute__.
>>>
>>> Note: this function may not be able to retrieve all attributes
>>> reported by dir(obj)
>>> """
>>> try:
>>> instance_dict = object.__getattribute__(obj, "__dict__")
>>> except AttributeError:
>>> pass
>>> else:
>>> if attr in instance_dict:
>>> return instance_dict[attr]
>>> for entry in getmro(obj.__class__):
>>> try:
>>> return entry.__dict__[attr]
>>> except AttributeError:
>>> pass
>> Second attempt with a default value parameter and correctly raising
>> AttributeError if not found:
>>
>> _sentinel = object()
>> def getattr_static(obj, attr, default=_sentinel):
>> """Retrieve attributes without triggering dynamic lookup via the
>> descriptor protocol, __getattr__ or __getattribute__.
>>
>> Note: this function may not be able to retrieve all attributes
>> reported by dir(obj)
>> """
>> try:
>> instance_dict = object.__getattribute__(obj, "__dict__")
>> except AttributeError:
>> pass
>> else:
>> if attr in instance_dict:
>> return instance_dict[attr]
>> for entry in getmro(obj.__class__):
>> try:
>> return entry.__dict__[attr]
>> except AttributeError:
>> pass
>> if default is not _sentinel:
>> return default
>> raise AttributeError(attr)
>>
>
> This doesn't correctly handle the case where obj is a type object or 
> obj uses __slots__.
>
> If I have time (currently on vacation with only intermittent internet 
> access) I'll provide an update.
>

I managed an updated version (with tests) during the flight home last 
night. I've attached it to issue 9732, along with a discussion of the 
caveats and ways of breaking it:

http://bugs.python.org/issue9732

Michael

> Michael
>
>
>> Cheers,
>> Nick.
>>
>
>


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