Print help caused by a command line error on stderr

b2/_internal/arg_parser.py

@@ -146,7 +146,9 @@ def _get_short_description(self) -> str:
         return ''
 
     def error(self, message):
-        self.print_help()
+        # Help printed because of a command line syntax error is part of the error
+        # report, not of the command output, hence it goes to stderr.
+        self.print_help(sys.stderr)
 
         self.exit(2, f'\n{self.prog}: error: {message}\n')
 

changelog.d/493.fixed.md

@@ -0,0 +1 @@
+Print help triggered by a command line syntax error on stderr instead of stdout, so that a mistyped command no longer pollutes redirected or piped output.

test/unit/console_tool/test_help.py

@@ -41,3 +41,29 @@ def test_help(b2_cli, flag, included, excluded, capsys):
             found.add(e)
     assert found.issuperset(included), f'expected {included!r} in {out!r}'
     assert found.isdisjoint(excluded), f'expected {excluded!r} not in {out!r}'
+
+
+@pytest.mark.parametrize(
+    'argv',
+    [
+        pytest.param(['--nonexistent-flag'], id='unrecognized-argument'),
+        pytest.param(['sync'], id='missing-required-arguments'),
+        pytest.param(['sync', '--nonexistent-flag'], id='unrecognized-subcommand-argument'),
+    ],
+)
+def test_help_on_command_line_error_goes_to_stderr(b2_cli, argv, capsys):
+    """Help printed because of a command line error belongs on stderr, not stdout."""
+    b2_cli.run(argv, expected_status=2, expected_stdout=None)
+
+    captured = capsys.readouterr()
+    assert captured.out == ''
+    assert 'error:' in captured.err
+    assert '-h, --help' in captured.err
+
+
+def test_help_on_request_goes_to_stdout(b2_cli, capsys):
+    b2_cli.run(['--help'], expected_stdout=None)
+
+    captured = capsys.readouterr()
+    assert '-h, --help' in captured.out
+    assert captured.err == ''