gh-117486: Improve behavior for user-defined AST subclasses (#118212)

Now, such classes will no longer require changes in Python 3.13 in the normal case.
The test suite for robotframework passes with no DeprecationWarnings under this PR.

I also added a new DeprecationWarning for the case where `_field_types` exists
but is incomplete, since that seems likely to indicate a user mistake.

1 files changed, 17 insertions, 14 deletions

diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index 1f0be45..11d59fa 100755
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -979,14 +979,9 @@ ast_type_init(PyObject *self, PyObject *args, PyObject *kw)
             goto cleanup;
         }
         if (field_types == NULL) {
-            if (PyErr_WarnFormat(
-                PyExc_DeprecationWarning, 1,
-                "%.400s provides _fields but not _field_types. "
-                "This will become an error in Python 3.15.",
-                Py_TYPE(self)->tp_name
-            ) < 0) {
-                res = -1;
-            }
+            // Probably a user-defined subclass of AST that lacks _field_types.
+            // This will continue to work as it did before 3.13; i.e., attributes
+            // that are not passed in simply do not exist on the instance.
             goto cleanup;
         }
         remaining_list = PySequence_List(remaining_fields);
@@ -997,12 +992,21 @@ ast_type_init(PyObject *self, PyObject *args, PyObject *kw)
             PyObject *name = PyList_GET_ITEM(remaining_list, i);
             PyObject *type = PyDict_GetItemWithError(field_types, name);
             if (!type) {
-                if (!PyErr_Occurred()) {
-                    PyErr_SetObject(PyExc_KeyError, name);
+                if (PyErr_Occurred()) {
+                    goto set_remaining_cleanup;
+                }
+                else {
+                    if (PyErr_WarnFormat(
+                        PyExc_DeprecationWarning, 1,
+                        "Field '%U' is missing from %.400s._field_types. "
+                        "This will become an error in Python 3.15.",
+                        name, Py_TYPE(self)->tp_name
+                    ) < 0) {
+                        goto set_remaining_cleanup;
+                    }
                 }
-                goto set_remaining_cleanup;
             }
-            if (_PyUnion_Check(type)) {
+            else if (_PyUnion_Check(type)) {
                 // optional field
                 // do nothing, we'll have set a None default on the class
             }
@@ -1026,8 +1030,7 @@ ast_type_init(PyObject *self, PyObject *args, PyObject *kw)
                     "This will become an error in Python 3.15.",
                     Py_TYPE(self)->tp_name, name
                 ) < 0) {
-                    res = -1;
-                    goto cleanup;
+                    goto set_remaining_cleanup;
                 }
             }
         }