gh-155358: Add test.support.check_immutable_type()

Check that sys types are immutable:

* type(sys.flags)
* type(sys.get_asyncgen_hooks())
* type(sys.getwindowsversion())
* type(sys.hash_info)
* type(sys.version_info)

Author: vstinner

Lib/test/support/__init__.py

@@ -3465,3 +3465,9 @@ def skip_on_low_desktop_heap_memory_subprocess(returncode):
     if returncode == STATUS_DLL_INIT_FAILED:
         raise unittest.SkipTest('gh-150436: DLL init failed, likely because '
                                 'of low desktop heap memory')
+
+
+def check_immutable_type(testcase, type):
+    regex = r'cannot set .* attribute of immutable type'
+    with testcase.assertRaisesRegex(TypeError, regex):
+        setattr(type, 'custom_attr', 123)

Lib/test/test_decimal.py

@@ -32,6 +32,7 @@
 import unittest
 import numbers
 import locale
+from test import support
 from test.support import (is_resource_enabled,
                           requires_IEEE_754, requires_docstrings,
                           check_disallow_instantiation)
@@ -5806,8 +5807,7 @@ def test_c_immutable_types(self):
         )
         for tp in types:
             with self.subTest(tp=tp):
-                with self.assertRaisesRegex(TypeError, "immutable"):
-                    tp.foo = 1
+                support.check_immutable_type(self, tp)
 
     def test_c_disallow_instantiation(self):
         ContextManager = type(C.localcontext())

Lib/test/test_itertools.py

@@ -1541,8 +1541,7 @@ def test_immutable_types(self):
         )
         for tp in dataset:
             with self.subTest(tp=tp):
-                with self.assertRaisesRegex(TypeError, "immutable"):
-                    tp.foobar = 1
+                support.check_immutable_type(self, tp)
 
 
 class TestExamples(unittest.TestCase):

Lib/test/test_sys.py

@@ -425,6 +425,7 @@ def test_getwindowsversion(self):
         self.assertEqual(v[2], v.build)
         self.assertEqual(v[3], v.platform)
         self.assertEqual(v[4], v.service_pack)
+        support.check_immutable_type(self, type(v))
 
         # This is how platform.py calls it. Make sure tuple
         #  still has 5 elements
@@ -690,6 +691,7 @@ def test_attributes(self):
             self.assertEqual(algo, 0)
         self.assertGreaterEqual(sys.hash_info.cutoff, 0)
         self.assertLess(sys.hash_info.cutoff, 8)
+        support.check_immutable_type(self, type(sys.hash_info))
 
         self.assertIsInstance(sys.maxsize, int)
         self.assertIsInstance(sys.maxunicode, int)
@@ -893,11 +895,13 @@ def assert_raise_on_new_sys_type(self, sys_attr):
         # sys.flags, sys.version_info, and sys.getwindowsversion.
         support.check_disallow_instantiation(self, type(sys_attr), sys_attr)
 
-    def test_sys_flags_no_instantiation(self):
+    def test_sys_flags_type(self):
         self.assert_raise_on_new_sys_type(sys.flags)
+        support.check_immutable_type(self, type(sys.flags))
 
-    def test_sys_version_info_no_instantiation(self):
+    def test_sys_version_info_type(self):
         self.assert_raise_on_new_sys_type(sys.version_info)
+        support.check_immutable_type(self, type(sys.version_info))
 
     def test_sys_getwindowsversion_no_instantiation(self):
         # Skip if not being run on Windows.
@@ -1954,6 +1958,7 @@ def test_asyncgen_hooks(self):
         cur = sys.get_asyncgen_hooks()
         self.assertIsNone(cur.firstiter)
         self.assertIsNone(cur.finalizer)
+        support.check_immutable_type(self, type(cur))
 
         # gh-118473
         with self.assertRaises(TypeError):
@@ -1997,6 +2002,7 @@ def write(self, s):
         self.assertEqual(out, b"")
         self.assertEqual(err, b"")
 
+
 @test.support.support_remote_exec_only
 @test.support.cpython_only
 class TestRemoteExec(unittest.TestCase):

Lib/test/test_xml_etree_c.py

@@ -194,8 +194,7 @@ def test_immutable_types(self):
         )
         for tp in dataset:
             with self.subTest(tp=tp):
-                with self.assertRaisesRegex(TypeError, "immutable"):
-                    tp.foo = 1
+                support.check_immutable_type(self, tp)
 
     @support.cpython_only
     def test_disallow_instantiation(self):