gh-156570: handle lazy warnings import in test helper

Lib/test/support/warnings_helper.py

@@ -173,7 +173,7 @@ def _filterwarnings(filters, quiet=False):
         registry.clear()
     # Because test_warnings swap the module, we need to look up in the
     # sys.modules dictionary.
-    wmod = sys.modules['warnings']
+    wmod = sys.modules.get('warnings', warnings)
     with wmod.catch_warnings(record=True) as w:
         # Set filter "always" to record all warnings.
         wmod.simplefilter("always")

Lib/test/test_support.py

@@ -84,6 +84,23 @@ def test_ignored_deprecations_are_silent(self):
             messages = [str(w.message) for w in warning_objs]
         self.assertEqual(len(messages), 0, messages)
 
+    def test_check_warnings_when_module_is_not_in_sys_modules(self):
+        warnings_module = sys.modules.pop("warnings", None)
+        original_catch_warnings = warnings_helper.warnings.catch_warnings
+        try:
+            def catch_warnings(*, record):
+                return original_catch_warnings(
+                    record=record, module=warnings_helper.warnings
+                )
+
+            warnings_helper.warnings.catch_warnings = catch_warnings
+            with warnings_helper.check_warnings(quiet=True):
+                pass
+        finally:
+            warnings_helper.warnings.catch_warnings = original_catch_warnings
+            if warnings_module is not None:
+                sys.modules["warnings"] = warnings_module
+
     def test_import_module(self):
         import_helper.import_module("ftplib")
         self.assertRaises(unittest.SkipTest,

Misc/NEWS.d/next/Tests/2026-08-29-20-15-00.gh-issue-156570.codex.rst

@@ -0,0 +1,2 @@
+Fix ``test.support.warnings_helper.check_warnings()`` failing with a
+``KeyError`` when lazy imports have not populated ``sys.modules``.