gh-156909: Fix crash in repr() of AST nodes without _fields

Lib/test/test_ast/test_ast.py

@@ -1165,6 +1165,20 @@ def test_repr_large_input_crash(self):
                                     r"Exceeds the limit \(\d+ digits\)"):
             repr(ast.Constant(value=eval(source)))
 
+    def test_repr_missing_fields_crash(self):
+        class FieldsMissingMeta(type):
+            def __getattribute__(self, name):
+                if name == "_fields":
+                    raise AttributeError
+                return super().__getattribute__(name)
+
+        class FieldsMissing(ast.AST, metaclass=FieldsMissingMeta):
+            def __init__(self):
+                pass
+
+        node = FieldsMissing()
+        self.assertEqual(repr(node), "FieldsMissing()")
+
     def test_tstring(self):
         # Test AST structure for simple t-string
         tree = ast.parse('t"Hello"')

Misc/NEWS.d/next/Core_and_Builtins/2026-09-11-11-34-02.gh-issue-156909.2Hgszb.rst

@@ -0,0 +1,2 @@
+Fix a crash in :func:`repr` of AST nodes when the AST node's type lacks the
+``_fields`` attribute. Patched by Shamil Abdulaev.

Parser/asdl_c.py

@@ -1498,6 +1498,11 @@ def visitModule(self, mod):
         return NULL;
     }
 
+    if (fields == NULL) {
+        Py_ReprLeave((PyObject *)self);
+        return PyUnicode_FromFormat("%s()", Py_TYPE(self)->tp_name);
+    }
+
     Py_ssize_t numfields = PySequence_Size(fields);
     if (numfields < 0) {
         Py_ReprLeave((PyObject *)self);