Bare `AttributeError` in `__getattr__` shows a garbled message with attribute suggestions

[johnslavik]

Bug report

Bug description:

Example instance of this bug in functools:

from functools import singledispatchmethod
 
class C:
    @singledispatchmethod
    def sdm(self, a):
        pass
        
C().sdm.__code__

Traceback gotten (-) vs. expected (+):

 Traceback (most recent call last):
   File "<python-input-3>", line 1, in <module>
     C().sdm.__code__
   File "/home/bswck/Python/cpython/Lib/functools.py", line 1096, in __getattr__
     raise AttributeError
-AttributeError: . Did you mean: '__call__'?
+AttributeError: '_singledispatchmethod_get' object has no attribute '__code__'. Did you mean: '__call__'?

The right fix is not to change functools, but to change traceback.TracebackException to handle bare AttributeErrors (complete the message). It already knows what the attribute is, otherwise it wouldn't compute suggestions.

Minimal reproducer:

class C:
    foobar = 1
    
    def __getattr__(self, _):
        raise AttributeError

C().foboar

Traceback (most recent call last):
  File "/home/bswck/Python/cpython/t.py", line 7, in <module>
    C().foboar
  File "/home/bswck/Python/cpython/t.py", line 5, in __getattr__
    raise AttributeError
AttributeError: . Did you mean: 'foobar'?

Backportable to 3.13 and 3.14.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-143811: Add helpful default error messages to AttributeError and NameError #143813

[johnslavik]

Reproduction of the same bug for NameError, much less likely to occur:

eggs = 42

try:
    eggz
except NameError as exc:
    exc.args = ()
    raise

❯ ./python t.py
Traceback (most recent call last):
  File "/home/bswck/Python/cpython/t.py", line 4, in <module>
    eggz
NameError: . Did you mean: 'eggs'?

[a12k]

I think we can do a fix in Lib/traceback.py TracebackException class to handle handle bare AttributeError/NameError with empty args. I'm thinking modify traceback.TracebackException to check for empty messages then reconstruct the standard Python error message using the available attributes.

I'd be happy to put together a quick PR for this @johnslavik if that approach seems reasonable..

[johnslavik]

I'd be happy to put together a quick PR for this @johnslavik if that approach seems reasonable..

Thanks @a12k! This time I've prepared a patch on the spot. A review is welcome once it's out of draft :)

[serhiy-storchaka]

First, we need to decide where to generate the error message. How is it so happened that we have the name and the object, but not the message? Ensuring that the message is not empty when this data is set is a possible option. Other option is to generate the error message dynamically. For now, generating it in traceback.py may be the right choice, but other options should also be considered.