[Python-Dev] Attribute lookup ambiguity

[Michael Foord]

On 20/03/2010 12:00, Pascal Chambon wrote:
> Michael Foord a écrit :
>>
>> On 19/03/2010 18:58, Pascal Chambon wrote:
>>> Hello
>>>
>>> I've already crossed a bunch of articles detailing python's 
>>> attribute lookup semantic (__dict__, descriptors, order of base 
>>> class traversing...), but I have never seen, so far, an explanation 
>>> of WHICH method did waht, exactly.
>>>
>>> I assumed that getattr(a, b) was the same as a.__getattribute__(b), 
>>> and that this __getattribute__ method (or the hidden routine 
>>> replacing it when we don't override it in our class) was in charge 
>>> of doing the whole job of traversing the object tree, checking 
>>> descriptors, binding methods, calling __getattr__ on failure etc.
>>>
>>> However, the test case below shows that __getattribute__ does NOT 
>>> call __getattr__ on failure. So it seems it's an upper levl 
>>> machinery, in getattr(), which is in chrge of that last action.
>>
>> Python 3 has the behavior you are asking for. It would be a backwards 
>> incompatible change to do it in Python 2 as __getattribute__ *not* 
>> calling __getattr__ is the documented behaviour.
>>
>> Python 3.2a0 (py3k:78770, Mar 7 2010, 20:32:50)
>> [GCC 4.2.1 (Apple Inc. build 5646) (dot 1)] on darwin
>> >>> class x:
>> ... def __getattribute__(s, name):
>> ... print ('__getattribute__', name)
>> ... raise AttributeError
>> ... def __getattr__(s, name):
>> ... print ('__getattr__', name)
>> ...
>> >>> a = x()
>> >>> a.b
>> __getattribute__ b
>> __getattr__ b
> I'm confused there, because the script you gave behaves the same in 
> python 2.6. And according to the doc, it's normal, getattr() reacts to 
> an AttributeError from __getattribute__, by calling __getattr__ :
>
> """
> Python 2.6.5 documentation
>
> object.__getattribute__(/self/, /name/)
>
>     Called unconditionally to implement attribute accesses for
>     instances of the class. If the class also defines __getattr__()
>     <http://docs.python.org/reference/datamodel.html#object.__getattr__>,
>     the latter will not be called unless __getattribute__()
>     <http://docs.python.org/reference/datamodel.html#object.__getattribute__>
>     either calls it explicitly or raises an AttributeError
>     <http://docs.python.org/library/exceptions.html#exceptions.AttributeError>.
>     This method should return the (computed) attribute value or raise
>     an AttributeError
>     <http://docs.python.org/library/exceptions.html#exceptions.AttributeError>
>     exception. In order to avoid infinite recursion in this method,
>     its implementation should always call the base class method with
>     the same name to access any attributes it needs, for example,
>     object.__getattribute__(self, name).
>
> """
>
> But the point which for me is still unclear, is : does the default 
> implementation of __getattribute__ (the one of "object") call 
> __getattr__ by himself, or does it rely on its caller for that, by 
> raising an AttributeError ? For Python2, it's blatantly the latter 
> case which is favoured, but since it looks like an implementation 
> detail at the moment, I propose we settle it (and document it) once 
> for all.

Ah right, my apologies. So it is still documented behaviour - 
__getattr__ is obviously called by the Python runtime and not by 
__getattribute__. (It isn't just by getattr as the same behaviour is 
shown when doing a normal attribute lookup and not via the getattr 
function.)

>
>>
>>
>> This list is not really an appropriate place to ask questions like 
>> this though, comp.lang.python would be better.
>>
>> All the best,
>>
>> Michael Fooord
> Sorry if I misposted, I just (wrongly ?) assumed that it was more an 
> undecided, implementation-specific point (since the doc gave possible 
> behaviours for __getattribute__, without precising which one was the 
> default one), and thus targetted the hands-in-core-code audience only.
>

Well, the documentation you pointed to specifies that __getattr__ will 
be called if __getattribute__ raises an AttributeError, it just doesn't 
specify that it is done by object.__getattribute__ (which it isn't).

Michael Foord

> Regards,
> Pascal
>
>


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