Michael Foord a écrit :

On 20/03/2010 12:00, Pascal Chambon wrote:

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.)

I really don't see the docs you're referring to ; until I tested myself, I think I had no obvious reasons to guess that __getattribute__ relied on the upper level caller instead of finishing the hard job himself.

Nick Coghlan a écrit :

Michael Foord wrote:

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).


And as for why not: because __getattribute__ implementations need to be
able to call object.__getattribute__ without triggering the fallback
behaviour.

Cheers,
Nick.

I guess there are cases in which it is beneficial indeed.

Michael Foord wrote:

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).

If __getattribute__ raises an exception, it won't get a chance to
do anything else, so something outside of __getattribute__ must
catch the AttributeError and calling __getattr__. So I think the
docs *are* specifying the behaviour here, if only by implication.

I don't follow you there - in my mind, the default __getattribute__ could simply have wrapped all its operations inside soem kind of "try..catch AttributeError:" mechanism, and thus been able to fallback to __getattr__ in any way.

If I sum it up properly the semantic is :
-A.obj and getattr(A, "obj") are exactly the same
-They trigger the calling of __getattribute__ on the object (or it's python core equivalent)
-By default, this __getattribute__ browse the whole object hierarchy according to well known rules (__dict__, type, type's ancestors..), handling descriptor protocols and the like. But it doesn't fallback to __getattr__ - it raises an AttributeError instead.
-getattr() falls back to __getattr__ if __getattribute__ fails
-customized __getattribute__ methods have the choice between calling __getattr__ by themselves, or delegating it to getattr() by raising an exception.

Wouldn't it be worth completing the doc with these point ? They really didn't seem obvious to me basically (even though, after analysis, some behaviours make more sense than others).
I might submit a patch.

regards,
Pascal

  