gh-154196: Improve AttributeError messages from unresolved lazy imports (GH-154688)

(cherry picked from commit 125ca26)

Co-authored-by: Bartosz Sławecki <bartosz@ilikepython.com>

Author: johnslavik

Lib/test/test_lazy_import/__init__.py

@@ -285,6 +285,23 @@ def test_lazy_import_type_attributes_accessible(self):
         proc = assert_python_ok("-c", code)
         self.assertIn(b"<built-in method resolve of lazy_import object at", proc.out)
 
+    @support.requires_subprocess()
+    def test_lazy_import_type_attribute_error_message(self):
+        """Check that LazyImportType attribute error message is helpful."""
+        code = textwrap.dedent("""
+            lazy import asyncio
+            try:
+                globals()["asyncio"].Task
+            except AttributeError as exc:
+                assert str(exc) == (
+                    "cannot access attribute 'Task' "
+                    "on unresolved lazy import 'asyncio'"
+                ), repr(str(exc))
+            else:
+                assert False, 'AttributeError is not raised'
+        """)
+        assert_python_ok("-c", code)
+
 
 class SyntaxRestrictionTests(LazyImportTestCase):
     """Tests for syntax restrictions on lazy imports."""

Misc/NEWS.d/next/Core_and_Builtins/2026-07-25-12-43-42.gh-issue-154196.0rAdob.rst

@@ -0,0 +1,2 @@
+Improve :exc:`AttributeError` messages from unresolved lazy imports. Patch
+by Bartosz Sławecki.

Objects/lazyimportobject.c

@@ -81,6 +81,29 @@ lazy_import_dealloc(PyObject *op)
     Py_TYPE(op)->tp_free(op);
 }
 
+/* Specialize the error message for failed attribute lookups. */
+static PyObject *
+lazy_import_getattro(PyObject *op, PyObject *name)
+{
+    PyObject *value = _PyObject_GenericGetAttrWithDict(op, name, NULL, /* suppress */1);
+    if (value == NULL) {
+        if (PyErr_Occurred()) {
+            // pass up non-AttributeError exception
+            return NULL;
+        }
+        PyObject *lz_name = _PyLazyImport_GetName(op);
+        if (lz_name == NULL) {
+            return NULL;
+        }
+        PyErr_Format(PyExc_AttributeError,
+                     "cannot access attribute %R on unresolved lazy import %R",
+                     name, lz_name);
+        Py_DECREF(lz_name);
+        return NULL;
+    }
+    return value;
+}
+
 static PyObject *
 lazy_import_name(PyLazyImportObject *m)
 {
@@ -149,6 +172,7 @@ PyTypeObject PyLazyImport_Type = {
     .tp_repr = lazy_import_repr,
     .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
     .tp_doc = lazy_import_doc,
+    .tp_getattro = lazy_import_getattro,
     .tp_traverse = lazy_import_traverse,
     .tp_clear = lazy_import_clear,
     .tp_methods = lazy_import_methods,