[issue45064] Raising AttributeError in descriptor decorator causing searching attribute in __mro__

[Raymond Hettinger]

Raymond Hettinger  added the comment:

To get a better insight into what is going on, see
https://docs.python.org/3/howto/descriptor.html#invocation-from-an-instance
  
The relevant text is, "So if __getattr__() exists, it is called whenever 
__getattribute__() raises AttributeError (either directly or in one of the 
descriptor calls)."

If needed, you can bypass the __getattr__ hook by calling 
object.__getattribute__ directly:

>>> object.__getattribute__(C(), 'test')
Traceback (most recent call last):
  File "", line 1, in 
  object.__getattribute__(c, 'test')
  File "/Users/raymond/Documents/tmp14.py", line 26, in __get__
raise AttributeError("Python bug?")
AttributeError: Python bug?

--
nosy: +rhettinger

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[issue45064] Raising AttributeError in descriptor decorator causing searching attribute in __mro__

[Yurii Karabas]

Yurii Karabas <1998uri...@gmail.com> added the comment:

Hi Mateusz,

It's not a bug, it's expected behavior.

__getattr__ called when the default attribute access fails with an 
AttributeError (either __getattribute__() raises an AttributeError because name 
is not an instance attribute or an attribute in the class tree for self; or 
__get__() of a name property raises AttributeError).

You can find more information here: 

https://docs.python.org/3/reference/datamodel.html?highlight=__getattr__#object.__getattr__

--
nosy: +uriyyo
resolution:  -> not a bug
stage:  -> resolved
status: open -> closed

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[issue45064] Raising AttributeError in descriptor decorator causing searching attribute in __mro__

[Mateusz]

New submission from Mateusz :

A descriptor that is raising AttributeError in __get__() causes that the 
Python's interpreter continues searching for attributes in __mro__ calling 
__getattr__() function in inherited classes.

Let's take a look for example script with this bug.


class A1:
def __getattr__(self, name):
print("A1 visited")
raise AttributeError(f"{self.__class__.__name__}: {name} not found.")

class A2(A1):
def __getattr__(self, name):
print("A2 visited")
super().__getattr__(name)


class A3(A2):
def __getattr__(self, name):
print("A3 visited")
super().__getattr__(name)


class B:
def __init__(self, f):
self.f = f

def __get__(self, obj, objtype=None):
raise AttributeError("Python bug?")

class C(A3):
@B
def test(self):
return 25

B is a decorator attached to C.test() and it is throwing AttributeError. When 
c.test() is performed it starts for walking through C.__mro__ and calling 
__getattr__() function in objects.

>>> from bug import C
>>> c = C()
>>> c.test()
A3 visited
A2 visited
A1 visited
Traceback (most recent call last):
  File "", line 1, in 
  File "/home/blooser/python-bug/bug.py", line 17, in __getattr__
super().__getattr__(name)
  File "/home/blooser/python-bug/bug.py", line 11, in __getattr__
super().__getattr__(name)
  File "/home/blooser/python-bug/bug.py", line 6, in __getattr__
raise AttributeError(f"{self.__class__.__name__}: {name} not found.")
AttributeError: C: test not found.

Changing error in B.__get__() to NameError:

class B:
def __init__(self, f):
self.f = f

def __get__(self, obj, objtype=None):
raise NameError("Python bug?")

causes it omits C.__mro__.

>>> from bug import C
>>> c = C()
>>> c.test()
Traceback (most recent call last):
  File "", line 1, in 
  File "/home/blooser/python-bug/bug.py", line 26, in __get__
raise NameError("Python bug?")
NameError: Python bug?

I'm thinking that it is expected behavior or is this a bug?

--
components: Interpreter Core
files: bug.py
messages: 400743
nosy: blooser
priority: normal
severity: normal
status: open
title: Raising AttributeError in descriptor decorator causing searching 
attribute in __mro__
type: behavior
versions: Python 3.11
Added file: https://bugs.python.org/file50248/bug.py

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