gh-153785: Generate AttributeError messages from context

Doc/library/exceptions.rst

@@ -215,9 +215,18 @@ The following exceptions are the exceptions that are usually raised.
 
       The object that was accessed for the named attribute.
 
+   When possible, :attr:`name` and :attr:`obj` are set automatically
+   for failed attribute lookups.
+
    .. versionchanged:: 3.10
       Added the :attr:`name` and :attr:`obj` attributes.
 
+   .. versionchanged:: next
+      The default error message is now generated from :attr:`name` and
+      :attr:`obj` when both attributes are set and the exception was
+      constructed with no positional arguments, or with a single positional
+      argument equal to :attr:`name`.
+
 .. exception:: EOFError
 
    Raised when the :func:`input` function hits an end-of-file condition (EOF)

Lib/test/test_exceptions.py

@@ -10,6 +10,7 @@
 from codecs import BOM_UTF8
 from itertools import product
 from textwrap import dedent
+from types import ModuleType
 
 from test.support import (captured_stderr, check_impl_detail,
                           cpython_only, gc_collect,
@@ -2047,6 +2048,49 @@ def blech(self):
             self.assertEqual("bluch", exc.name)
             self.assertEqual(obj, exc.obj)
 
+    def _test_generated_message(self, obj, name):
+        with self.assertRaises(AttributeError) as cm:
+            getattr(obj, name)
+        self.assertEqual(str(cm.exception),
+                         f"{obj!r} has no attribute {name!r}")
+
+    def test_getattr_error_message(self):
+        class RaiseWithName:
+            def __getattr__(self, name):
+                raise AttributeError(name)
+
+        class BareRaise:
+            def __getattr__(self, name):
+                raise AttributeError
+
+        class RaiseCustom:
+            def __getattr__(self, name):
+                raise AttributeError("custom")
+
+        self._test_generated_message(RaiseWithName(), "missing1")
+        self._test_generated_message(BareRaise(), "missing2")
+        self._test_generated_message(RaiseCustom(), "custom")
+
+    def test_module_getattr_error_message(self):
+        mod1 = ModuleType("raisewithname")
+        def raise_with_name(name):
+            raise AttributeError(name)
+        mod1.__getattr__ = raise_with_name
+
+        mod2 = ModuleType("bareraise")
+        def bare_raise(name):
+            raise AttributeError
+        mod2.__getattr__ = bare_raise
+
+        mod3 = ModuleType("bareraise")
+        def raise_custom(name):
+            raise AttributeError("custom")
+        mod3.__getattr__ = raise_custom
+
+        self._test_generated_message(mod1, "missing3")
+        self._test_generated_message(mod2, "missing4")
+        self._test_generated_message(mod3, "custom")
+
     # Note: name suggestion tests live in `test_traceback`.
 
 

Misc/NEWS.d/next/Core_and_Builtins/2026-07-16-02-50-01.gh-issue-153785.fJqPKC.rst

@@ -0,0 +1,4 @@
+The default error message is now generated from ``name`` and ``obj`` attributes
+of :exc:`AttributeError` when both are set and the exception was constructed with
+no positional arguments, or with a single positional argument equal to ``name``.
+Patch by Bartosz Sławecki.

Objects/exceptions.c

@@ -2702,6 +2702,36 @@ AttributeError_dealloc(PyObject *self)
     Py_TYPE(self)->tp_free(self);
 }
 
+static PyObject *
+AttributeError_str(PyObject *op)
+{
+    PyAttributeErrorObject *self = PyAttributeErrorObject_CAST(op);
+    PyObject *arg, *obj = NULL, *name;
+
+    Py_BEGIN_CRITICAL_SECTION(self);
+    if (
+        self->obj && self->name && PyUnicode_Check(self->name)
+        && ((PyTuple_GET_SIZE(self->args) == 1
+             && PyUnicode_Check(arg = PyTuple_GET_ITEM(self->args, 0))
+             && PyUnicode_Equal(arg, self->name))
+            || PyTuple_GET_SIZE(self->args) == 0)
+    ) {
+        obj = Py_NewRef(self->obj);
+        name = Py_NewRef(self->name);
+    }
+    Py_END_CRITICAL_SECTION();
+
+    if (!obj) {
+        return BaseException_str(op);
+    }
+
+    PyObject *result = PyUnicode_FromFormat("%R has no attribute '%U'",
+                                            obj, name);
+    Py_DECREF(obj);
+    Py_DECREF(name);
+    return result;
+}
+
 static int
 AttributeError_traverse(PyObject *op, visitproc visit, void *arg)
 {
@@ -2770,7 +2800,7 @@ static PyMethodDef AttributeError_methods[] = {
 ComplexExtendsException(PyExc_Exception, AttributeError,
                         AttributeError, 0,
                         AttributeError_methods, AttributeError_members,
-                        0, BaseException_str, 0, "Attribute not found.");
+                        0, AttributeError_str, 0, "Attribute not found.");
 
 /*
  *    SyntaxError extends Exception