bpo-44089: Allow subclassing of ``csv.Error`` (GH-26008)

* fix subclass error

* Update 2021-05-09-22-52-34.bpo-44089.IoANsN.rst

Lib/test/test_csv.py

@@ -1237,6 +1237,9 @@ class MiscTestCase(unittest.TestCase):
         extra = {'__doc__', '__version__'}
         support.check__all__(self, csv, ('csv', '_csv'), extra=extra)
 
+    def test_subclassable(self):
+        # issue 44089
+        class Foo(csv.Error): ...
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Library/2021-05-09-22-52-34.bpo-44089.IoANsN.rst

@@ -0,0 +1,2 @@
+Allow subclassing ``csv.Error`` in 3.10 (it was allowed in 3.9 and earlier but
+was disallowed in early versions of 3.10).

Modules/_csv.c

@@ -1515,7 +1515,7 @@ static PyType_Slot error_slots[] = {
 
 PyType_Spec error_spec = {
     .name = "_csv.Error",
-    .flags = Py_TPFLAGS_DEFAULT,
+    .flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
     .slots = error_slots,
 };