bpo-43857: Improve the AttributeError message when deleting a missing attribute

[geryogam]

https://bugs.python.org/issue43857

#88023

[github-actions]

This PR is stale because it has been open for 30 days with no activity.

[JelleZijlstra]

I agree with this idea. Could you add a NEWS entry and a unit test that exercises this message?

(I promise I'm not stalking you, I just opened page 24 of the open PRs and found this one :) .)

[geryogam]

Thanks @JelleZijlstra, but please don’t merge right now as some tests are failing. I am going to work on it in the next few hours. I’ll let you know when it works.

[geryogam]

Not only objects but also type objects have inconsistent AttributeError messages between missing attribute retrieval and deletion:

$ python3
Python 3.9.12 (main, Mar 26 2022, 15:52:10) 
[Clang 13.0.0 (clang-1300.0.29.30)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> class A: pass
... 
>>> A.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'A' has no attribute 'x'
>>> del A.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: x
>>> A().x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'A' object has no attribute 'x'
>>> del A().x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: x

Compare that with PyPy for which the messages are consistent (well almost, 'type' object has no attribute 'x' should be type object 'A' has no attribute 'x' for del A.x):

$ pypy3
Python 3.7.13 (7e0ae751533460d5f89f3ac48ce366d8642d1db5, Apr 26 2022, 09:31:20)
[PyPy 7.3.9 with GCC Apple LLVM 13.0.0 (clang-1300.0.29.30)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>> class A: pass
>>>> 
>>>> A.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: type object 'A' has no attribute 'x'
>>>> del A.x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'type' object has no attribute 'x'
>>>> A().x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'A' object has no attribute 'x'
>>>> del A().x
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'A' object has no attribute 'x'

Following @markshannon’s PR #28802 for Python 3.11, the message fix of this line in Objects/object.c provided at the early stage of this PR no longer works for objects but only for type objects:

      if (res < 0 && PyErr_ExceptionMatches(PyExc_KeyError))
-         PyErr_SetObject(PyExc_AttributeError, name);
+         PyErr_Format(PyExc_AttributeError,
+                      "'%.100s' object has no attribute '%U'",
+                      tp->tp_name, name);

To fix the message for objects, we should also update this line in Objects/dictobject.c:

-             PyErr_SetObject(PyExc_AttributeError, name);
+             PyErr_Format(PyExc_AttributeError,
+                          "'%.100s' object has no attribute '%U'",
+                          Py_TYPE(obj)->tp_name, name);

That way we get the same messages as in PyPy.

Finally to solve the last message inconsistency ('type' object has no attribute 'x' should be type object 'A' has no attribute 'x' for del A.x), I replaced the last fix in Objects/object.c with this fix:

-     if (res < 0 && PyErr_ExceptionMatches(PyExc_KeyError))
-         PyErr_SetObject(PyExc_AttributeError, name);
+     if (res < 0 && PyErr_ExceptionMatches(PyExc_KeyError)) {
+         if (PyType_IsSubtype(tp, &PyType_Type)) {
+             PyErr_Format(PyExc_AttributeError,
+                          "type object '%.50s' has no attribute '%U'",
+                          ((PyTypeObject*)obj)->tp_name, name);
+         }
+         else {
+             PyErr_Format(PyExc_AttributeError,
+                          "'%.100s' object has no attribute '%U'",
+                          tp->tp_name, name);
+         }
+     }

[geryogam]

@JelleZijlstra Done, all tests are passing. The PR is ready.

[geryogam]

Thanks again @JelleZijlstra!

(I promise I'm not stalking you, I just opened page 24 of the open PRs and found this one :) .)

No worries. And even if you were stalking my PRs I would not complain, quite the opposite =)