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\test_object_model.py", line 34, in <module>
print object.__getattribute__(B(), "obj") # DOES NOT CALL
__getattr__() !!!???
AttributeError: 'B' object has no attribute 'obj'
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