bpo-42955: Add sys.modules_names

Doc/library/sys.rst

@@ -153,10 +153,12 @@ always available.
 
 .. data:: builtin_module_names
 
-   A tuple of strings giving the names of all modules that are compiled into this
+   A tuple of strings containing the names of all modules that are compiled into this
    Python interpreter.  (This information is not available in any other way ---
    ``modules.keys()`` only lists the imported modules.)
 
+   See also the :attr:`sys.module_names` list.
+
 
 .. function:: call_tracing(func, args)
 
@@ -1060,6 +1062,24 @@ always available.
         This is still called as a fallback if a :data:`meta_path` entry doesn't
         have a :meth:`~importlib.abc.MetaPathFinder.find_spec` method.
 
+.. data:: module_names
+
+   A frozenset of strings containing the names of standard library modules.
+
+   It is the same on all platforms. Modules which are not available on
+   some platforms and modules disabled at Python build are also listed.
+   All module kinds are listed: pure Python, built-in, frozen and extension
+   modules. Test modules are excluded.
+
+   For packages, only sub-packages are listed, not sub-modules. For example,
+   ``concurrent`` package and ``concurrent.futures`` sub-package are listed,
+   but not ``concurrent.futures.base`` sub-module.
+
+   See also the :attr:`sys.builtin_module_names` list.
+
+   .. versionchanged:: 3.10
+
+
 .. data:: modules
 
    This is a dictionary that maps module names to modules which have already been

Doc/whatsnew/3.10.rst

@@ -354,6 +354,10 @@ Add :data:`sys.orig_argv` attribute: the list of the original command line
 arguments passed to the Python executable.
 (Contributed by Victor Stinner in :issue:`23427`.)
 
+Add :data:`sys.module_names`, containing the list of the standard library
+module names.
+(Contributed by Victor Stinner in :issue:`42955`.)
+
 threading
 ---------
 

Lib/test/test_capi.py

@@ -569,12 +569,23 @@ def check_fatal_error(self, code, expected, not_expected=()):
         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):
 

Lib/test/test_faulthandler.py

@@ -334,8 +334,9 @@ def test_disable(self):
     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 @@ def test_dump_ext_modules(self):
         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

Lib/test/test_sys.py

@@ -986,6 +986,11 @@ def test_orig_argv(self):
         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):

Misc/NEWS.d/next/Library/2021-01-18-11-59-46.bpo-42955.CSWLC9.rst

@@ -0,0 +1,2 @@
+Add :data:`sys.module_names`, containing the list of the standard library
+module names. Patch by Victor Stinner.