Add _showwarnmsg() and _formatwarnmsg() to warnings

Issue #26568: add new  _showwarnmsg() and _formatwarnmsg() functions to the
warnings module.

The C function warn_explicit() now calls warnings._showwarnmsg() with a
warnings.WarningMessage as parameter, instead of calling warnings.showwarning()
with multiple parameters.

_showwarnmsg() calls warnings.showwarning() if warnings.showwarning() was
replaced. Same for _formatwarnmsg(): call warnings.formatwarning() if it was
replaced.

1 files changed, 11 insertions, 0 deletions

diff --git a/Lib/test/test_warnings/__init__.py b/Lib/test/test_warnings/__init__.py
index cea9c57..70eae4c 100644
--- a/Lib/test/test_warnings/__init__.py
+++ b/Lib/test/test_warnings/__init__.py
@@ -651,6 +651,17 @@ class _WarningsTests(BaseTest, unittest.TestCase):
                 result = stream.getvalue()
         self.assertIn(text, result)
 
+    def test_showwarnmsg_missing(self):
+        # Test that _showwarnmsg() missing is okay.
+        text = 'del _showwarnmsg test'
+        with original_warnings.catch_warnings(module=self.module):
+            self.module.filterwarnings("always", category=UserWarning)
+            del self.module._showwarnmsg
+            with support.captured_output('stderr') as stream:
+                self.module.warn(text)
+                result = stream.getvalue()
+        self.assertIn(text, result)
+
     def test_showwarning_not_callable(self):
         with original_warnings.catch_warnings(module=self.module):
             self.module.filterwarnings("always", category=UserWarning)