bpo-42111: Make the xxlimited module an example of best extension module practices

Lib/test/test_xxlimited.py

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+import unittest
+from test.support import import_helper
+import types
+
+xxlimited = import_helper.import_module('xxlimited')
+xxlimited_35 = import_helper.import_module('xxlimited_35')
+
+
+class CommonTests:
+    module: types.ModuleType
+
+    def test_xxo_new(self):
+        xxo = self.module.Xxo()
+
+    def test_xxo_attributes(self):
+        xxo = self.module.Xxo()
+        with self.assertRaises(AttributeError):
+            xxo.foo
+        with self.assertRaises(AttributeError):
+            del xxo.foo
+
+        xxo.foo = 1234
+        self.assertEqual(xxo.foo, 1234)
+
+        del xxo.foo
+        with self.assertRaises(AttributeError):
+            xxo.foo
+
+    def test_foo(self):
+        # the foo function adds 2 numbers
+        self.assertEqual(self.module.foo(1, 2), 3)
+
+    def test_str(self):
+        self.assertTrue(issubclass(self.module.Str, str))
+        self.assertIsNot(self.module.Str, str)
+
+        custom_string = self.module.Str("abcd")
+        self.assertEqual(custom_string, "abcd")
+        self.assertEqual(custom_string.upper(), "ABCD")
+
+    def test_new(self):
+        xxo = self.module.new()
+        self.assertEqual(xxo.demo("abc"), "abc")
+
+
+class TestXXLimited(CommonTests, unittest.TestCase):
+    module = xxlimited
+
+    def test_xxo_demo(self):
+        xxo = self.module.Xxo()
+        other = self.module.Xxo()
+        self.assertEqual(xxo.demo("abc"), "abc")
+        self.assertEqual(xxo.demo(xxo), xxo)
+        self.assertEqual(xxo.demo(other), other)
+        self.assertEqual(xxo.demo(0), None)
+
+    def test_error(self):
+        with self.assertRaises(self.module.Error):
+            raise self.module.Error
+
+
+class TestXXLimited35(CommonTests, unittest.TestCase):
+    module = xxlimited_35
+
+    def test_xxo_demo(self):
+        xxo = self.module.Xxo()
+        other = self.module.Xxo()
+        self.assertEqual(xxo.demo("abc"), "abc")
+        self.assertEqual(xxo.demo(0), None)
+
+    def test_roj(self):
+        # the roj function always fails
+        with self.assertRaises(SystemError):
+            self.module.roj(0)
+
+    def test_null(self):
+        null1 = self.module.Null()
+        null2 = self.module.Null()
+        self.assertNotEqual(null1, null2)

Misc/NEWS.d/next/C API/2020-10-21-18-43-06.bpo-42111.9pvtrc.rst

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+Update the ``xxlimited`` module to be a better example of how to use the
+limited C API.