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 "<stdin>", line 1, in <module>
  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 "<stdin>", line 1, in <module>
  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?

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__

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 "<pyshell#6>", line 1, in <module>
      object.__getattribute__(c, 'test')
      File "/Users/raymond/Documents/tmp14.py", line 26, in __get__
        raise AttributeError("Python bug?")
    AttributeError: Python bug?