[Python-Dev] Attribute lookup ambiguity (original) (raw)

Michael Foord fuzzyman at voidspace.org.uk
Fri Mar 19 21:23:43 CET 2010

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

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

Is that on purpose ? Considering that getattribute (at lest, object.getattribute) does 90% of the hard job, why are these 10% left ? Can we find somewhere the details of "who must do what" when customizing attribute access ? Shouldn't we inform people about the fact that getattribute isn't sufficient in itself to lookup an attribute ? Thanks for the attention, regards, Pascal

======= INPUT ======= class A(object): def getattribute(self, name): print "A getattribute", name return object.getattribute(self, name) def getattr(self, name): print "A getattr", name return "hello A" class B(A): def getattribute(self, name): print "B getattribute", name return A.getattribute(self, name) def getattr(self, name): print "B getattr", name return "hello B" print A().obj print "---" print B().obj print "---" print getattr(B(), "obj") print "-----" print object.getattribute(B(), "obj") # DOES NOT CALL getattr() !!! =========== OUTPUT =========== A getattribute obj A getattr obj hello A --- B getattribute obj A getattribute obj B getattr obj hello B --- B getattribute obj A getattribute obj B getattr obj hello B ----- Traceback (most recent call last): File "C:\Users\Pakal\Desktop\testobjectmodel.py", line 34, in print object.getattribute(B(), "obj") # DOES NOT CALL getattr() !!!??? AttributeError: 'B' object has no attribute 'obj'

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