gh-101640: Make argparse _print_message catch any write error (#101802)

* In particular, don't exit when trying to print to stderr = None.
* Add tests

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>

Lib/test/test_argparse.py

@@ -1,5 +1,7 @@
 # Author: Steven J. Bethard <steven.bethard@gmail.com>.
 
+import contextlib
+import functools
 import inspect
 import io
 import operator
@@ -35,6 +37,35 @@ class StdIOBuffer(io.TextIOWrapper):
         return self.buffer.raw.getvalue().decode('utf-8')
 
 
+class StdStreamTest(unittest.TestCase):
+
+    def test_skip_invalid_stderr(self):
+        parser = argparse.ArgumentParser()
+        with (
+            contextlib.redirect_stderr(None),
+            mock.patch('argparse._sys.exit')
+        ):
+            parser.exit(status=0, message='foo')
+
+    def test_skip_invalid_stdout(self):
+        parser = argparse.ArgumentParser()
+        for func in (
+            parser.print_usage,
+            parser.print_help,
+            functools.partial(parser.parse_args, ['-h'])
+        ):
+            with (
+                self.subTest(func=func),
+                contextlib.redirect_stdout(None),
+                # argparse uses stderr as a fallback
+                StdIOBuffer() as mocked_stderr,
+                contextlib.redirect_stderr(mocked_stderr),
+                mock.patch('argparse._sys.exit'),
+            ):
+                func()
+                self.assertRegex(mocked_stderr.getvalue(), r'usage:')
+
+
 class TestCase(unittest.TestCase):
 
     def setUp(self):