gh-135797: Add _field_types to abstract AST classes

Classes without fields never got a _field_types dictionary, so the
attribute was missing on ast.AST and on the abstract classes such as
ast.mod, while typeshed declares it on ast.AST.  make_type() now sets an
empty dictionary on every class it creates, and ast.AST gets one too;
add_ast_annotations() keeps filling in the concrete classes.

The constructor relied on the missing attribute to detect user subclasses
that declare _fields but no _field_types, and those must keep working as
before 3.13.  An empty dictionary is treated the same way now, otherwise
they would start failing with a missing-field TypeError.

Author: Wojusensei

Doc/library/ast.rst

@@ -75,8 +75,9 @@ Node classes
 
    .. attribute:: _field_types
 
-      The :attr:`!_field_types` attribute on each concrete class is a dictionary
-      mapping field names (as also listed in :attr:`_fields`) to their types.
+      The :attr:`!_field_types` attribute on each class is a dictionary mapping
+      field names (as also listed in :attr:`_fields`) to their types.  Classes
+      without fields, such as the abstract classes, have an empty dictionary.
 
       .. doctest::
 
@@ -85,6 +86,10 @@ Node classes
 
       .. versionadded:: 3.13
 
+      .. versionchanged:: 3.16
+         The attribute is now also available on abstract classes and on other
+         classes without fields.
+
    .. attribute:: lineno
                   col_offset
                   end_lineno

Lib/test/test_ast/test_ast.py

@@ -438,6 +438,18 @@ def test_field_attr_existence(self):
                 if isinstance(x, ast.AST):
                     self.assertIs(type(x._fields), tuple)
 
+    def test_field_types_attr_existence(self):
+        # gh-135797: every AST class has a _field_types dictionary whose keys
+        # are the names listed in _fields.  Abstract classes have no fields
+        # and thus an empty dictionary, just like their empty _fields tuple.
+        for name, cls in ast.__dict__.items():
+            if isinstance(cls, type) and issubclass(cls, ast.AST):
+                with self.subTest(cls=name):
+                    self.assertIsInstance(cls._field_types, dict)
+                    self.assertEqual(set(cls._field_types), set(cls._fields))
+        self.assertEqual(ast.AST._field_types, {})
+        self.assertEqual(ast.mod._field_types, {})
+
     def test_dynamic_attr(self):
         for name, item in ast.__dict__.items():
             # constructor has a different signature

Misc/NEWS.d/next/Core_and_Builtins/2026-10-17-02-02-31.gh-issue-135797.Qs7vKd.rst

@@ -0,0 +1,3 @@
+Add an empty ``_field_types`` dictionary to :class:`ast.AST` and to the
+abstract :mod:`ast` classes that have no fields, such as ``ast.mod`` and
+``ast.stmt``.  The attribute was previously only set on classes with fields.

Parser/asdl_c.py

@@ -1134,10 +1134,14 @@ def visitModule(self, mod):
             res = -1;
             goto cleanup;
         }
-        if (field_types == NULL) {
-            // Probably a user-defined subclass of AST that lacks _field_types.
+        if (field_types == NULL ||
+            (PyDict_Check(field_types) && PyDict_GET_SIZE(field_types) == 0))
+        {
+            // Probably a user-defined subclass of AST that lacks _field_types,
+            // or that inherits the empty dictionary of an abstract AST class.
             // 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.
+            Py_XDECREF(field_types);
             goto cleanup;
         }
         remaining_list = PySequence_List(remaining_fields);
@@ -1690,7 +1694,7 @@ def visitModule(self, mod):
 make_type(struct ast_state *state, const char *type, PyObject* base,
           const char* const* fields, int num_fields, const char *doc)
 {
-    PyObject *fnames, *result;
+    PyObject *fnames, *field_types, *result;
     int i;
     fnames = PyTuple_New(num_fields);
     if (!fnames) return NULL;
@@ -1710,6 +1714,23 @@ def visitModule(self, mod):
                     state->ast,
                     state->__doc__, doc);
     Py_DECREF(fnames);
+    if (result == NULL) {
+        return NULL;
+    }
+    // Every AST class owns a _field_types dictionary.  Classes without fields,
+    // such as the abstract sum types, simply have an empty one.  The
+    // dictionaries of concrete classes are filled in by add_ast_annotations().
+    field_types = PyDict_New();
+    if (field_types == NULL) {
+        Py_DECREF(result);
+        return NULL;
+    }
+    if (PyObject_SetAttrString(result, "_field_types", field_types) < 0) {
+        Py_DECREF(field_types);
+        Py_DECREF(result);
+        return NULL;
+    }
+    Py_DECREF(field_types);
     return result;
 }
 
@@ -1839,16 +1860,26 @@ def visitModule(self, mod):
 
 static int add_ast_fields(struct ast_state *state)
 {
-    PyObject *empty_tuple;
+    PyObject *empty_tuple, *empty_dict;
     empty_tuple = PyTuple_New(0);
-    if (!empty_tuple ||
-        PyObject_SetAttrString(state->AST_type, "_fields", empty_tuple) < 0 ||
+    if (!empty_tuple) {
+        return -1;
+    }
+    empty_dict = PyDict_New();
+    if (empty_dict == NULL) {
+        Py_DECREF(empty_tuple);
+        return -1;
+    }
+    if (PyObject_SetAttrString(state->AST_type, "_fields", empty_tuple) < 0 ||
+        PyObject_SetAttrString(state->AST_type, "_field_types", empty_dict) < 0 ||
         PyObject_SetAttrString(state->AST_type, "__match_args__", empty_tuple) < 0 ||
         PyObject_SetAttrString(state->AST_type, "_attributes", empty_tuple) < 0) {
-        Py_XDECREF(empty_tuple);
+        Py_DECREF(empty_tuple);
+        Py_DECREF(empty_dict);
         return -1;
     }
     Py_DECREF(empty_tuple);
+    Py_DECREF(empty_dict);
     return 0;
 }