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

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

Co-authored-by: Terry Jan Reedy <[email protected]>

Author: arhadthedev

Lib/argparse.py

@@ -2605,9 +2605,11 @@ def print_help(self, file=None):
 
     def _print_message(self, message, file=None):
         if message:
-            if file is None:
-                file = _sys.stderr
-            file.write(message)
+            file = file or _sys.stderr
+            try:
+                file.write(message)
+            except (AttributeError, OSError):
+                pass
 
     # ===============
     # Exiting methods

Lib/test/test_argparse.py

@@ -1,5 +1,7 @@
 # Author: Steven J. Bethard <[email protected]>.
 
+import contextlib
+import functools
 import inspect
 import io
 import operator
@@ -35,6 +37,35 @@ def getvalue(self):
         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):

Misc/NEWS.d/next/Library/2023-02-09-22-24-34.gh-issue-101640.oFuEpB.rst

@@ -0,0 +1 @@
+:class:`argparse.ArgumentParser` now catches errors when writing messages, such as when :data:`sys.stderr` is ``None``. Patch by Oleg Iarygin.