gh-104265 Disallow instantiation of _csv.Reader and _csv.Writer

Lib/test/test_csv.py

@@ -7,10 +7,11 @@
 from io import StringIO
 from tempfile import TemporaryFile
 import csv
+import _csv
 import gc
 import pickle
 from test import support
-from test.support import warnings_helper
+from test.support import warnings_helper, check_disallow_instantiation
 from itertools import permutations
 from textwrap import dedent
 from collections import OrderedDict
@@ -1430,5 +1431,19 @@ def test_subclassable(self):
         # issue 44089
         class Foo(csv.Error): ...
 
+    def test_reader_disallow_instantiation(self):
+        check_disallow_instantiation(self, _csv.Reader)
+
+    def test_writer_disallow_instantiation(self):
+        check_disallow_instantiation(self, _csv.Writer)
+
+    def test_reader_not_basetype(self):
+        with self.assertRaisesRegex(TypeError, "type '_csv.reader' is not an acceptable base type"):
+            class Foo(_csv.Reader): pass
+
+    def test_writer_not_basetype(self):
+        with self.assertRaisesRegex(TypeError, "type '_csv.writer' is not an acceptable base type"):
+            class Foo(_csv.Writer): pass
+
 if __name__ == '__main__':
     unittest.main()

Modules/_csv.c

@@ -999,8 +999,8 @@ static PyType_Slot Reader_Type_slots[] = {
 PyType_Spec Reader_Type_spec = {
     .name = "_csv.reader",
     .basicsize = sizeof(ReaderObj),
-    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC |
-              Py_TPFLAGS_IMMUTABLETYPE),
+    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
+              Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_DISALLOW_INSTANTIATION),
     .slots = Reader_Type_slots
 };
 
@@ -1430,8 +1430,8 @@ static PyType_Slot Writer_Type_slots[] = {
 PyType_Spec Writer_Type_spec = {
     .name = "_csv.writer",
     .basicsize = sizeof(WriterObj),
-    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE | Py_TPFLAGS_HAVE_GC |
-              Py_TPFLAGS_IMMUTABLETYPE),
+    .flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC |
+              Py_TPFLAGS_IMMUTABLETYPE | Py_TPFLAGS_DISALLOW_INSTANTIATION),
     .slots = Writer_Type_slots,
 };