bpo-42955: Add sys.modules_names (GH-24238)

Add sys.module_names, containing the list of the standard library
module names.

3 files changed, 24 insertions, 6 deletions

diff --git a/Lib/test/test_capi.py b/Lib/test/test_capi.py
index 67175cd044a..5f5c0d038d9 100644
--- a/Lib/test/test_capi.py
+++ b/Lib/test/test_capi.py
@@ -569,12 +569,23 @@ class CAPITest(unittest.TestCase):
         self.assertEqual(len(modules), total)
 
     def test_fatal_error(self):
+        # By default, stdlib extension modules are ignored,
+        # but not test modules.
         expected = ('_testcapi',)
-        not_expected = ('sys', 'builtins', '_imp', '_thread', '_weakref',
-                        '_io', 'marshal', '_signal', '_abc')
-        code = 'import _testcapi; _testcapi.fatal_error(b"MESSAGE")'
+        not_expected = ('sys',)
+        code = 'import _testcapi, sys; _testcapi.fatal_error(b"MESSAGE")'
         self.check_fatal_error(code, expected, not_expected)
 
+        # Mark _testcapi as stdlib module, but not sys
+        expected = ('sys',)
+        not_expected = ('_testcapi',)
+        code = textwrap.dedent('''
+            import _testcapi, sys
+            sys.module_names = frozenset({"_testcapi"})
+            _testcapi.fatal_error(b"MESSAGE")
+        ''')
+        self.check_fatal_error(code, expected)
+
 
 class TestPendingCalls(unittest.TestCase):
 
diff --git a/Lib/test/test_faulthandler.py b/Lib/test/test_faulthandler.py
index b4a654f8a9c..02077a69bb4 100644
--- a/Lib/test/test_faulthandler.py
+++ b/Lib/test/test_faulthandler.py
@@ -334,8 +334,9 @@ class FaultHandlerTests(unittest.TestCase):
     def test_dump_ext_modules(self):
         code = """
             import faulthandler
-            # _testcapi is a test module and not considered as a stdlib module
-            import _testcapi
+            import sys
+            # Don't filter stdlib module names
+            sys.module_names = frozenset()
             faulthandler.enable()
             faulthandler._sigsegv()
             """
@@ -346,7 +347,8 @@ class FaultHandlerTests(unittest.TestCase):
         if not match:
             self.fail(f"Cannot find 'Extension modules:' in {stderr!r}")
         modules = set(match.group(1).strip().split(', '))
-        self.assertIn('_testcapi', modules)
+        for name in ('sys', 'faulthandler'):
+            self.assertIn(name, modules)
 
     def test_is_enabled(self):
         orig_stderr = sys.stderr
diff --git a/Lib/test/test_sys.py b/Lib/test/test_sys.py
index 3af5b117aff..729b8667fc8 100644
--- a/Lib/test/test_sys.py
+++ b/Lib/test/test_sys.py
@@ -986,6 +986,11 @@ class SysModuleTest(unittest.TestCase):
         self.assertEqual(proc.stdout.rstrip().splitlines(), expected,
                          proc)
 
+    def test_module_names(self):
+        self.assertIsInstance(sys.module_names, frozenset)
+        for name in sys.module_names:
+            self.assertIsInstance(name, str)
+
 
 @test.support.cpython_only
 class UnraisableHookTest(unittest.TestCase):