gh-139946: distinguish stdout or stderr when colorizing output in argparse

Lib/argparse.py

@@ -191,10 +191,10 @@ def __init__(
         self._whitespace_matcher = _re.compile(r'\s+', _re.ASCII)
         self._long_break_matcher = _re.compile(r'\n\n\n+')
 
-    def _set_color(self, color):
+    def _set_color(self, color, *, file=None):
         from _colorize import can_colorize, decolor, get_theme
 
-        if color and can_colorize():
+        if color and can_colorize(file=file):
             self._theme = get_theme(force_color=True).argparse
             self._decolor = decolor
         else:
@@ -2693,14 +2693,16 @@ def _check_value(self, action, value):
     # Help-formatting methods
     # =======================
 
-    def format_usage(self):
-        formatter = self._get_formatter()
+    def format_usage(self, formatter=None):
+        if formatter is None:
+            formatter = self._get_formatter()
         formatter.add_usage(self.usage, self._actions,
                             self._mutually_exclusive_groups)
         return formatter.format_help()
 
-    def format_help(self):
-        formatter = self._get_formatter()
+    def format_help(self, formatter=None):
+        if formatter is None:
+            formatter = self._get_formatter()
 
         # usage
         formatter.add_usage(self.usage, self._actions,
@@ -2722,9 +2724,9 @@ def format_help(self):
         # determine help from format above
         return formatter.format_help()
 
-    def _get_formatter(self):
+    def _get_formatter(self, file=None):
         formatter = self.formatter_class(prog=self.prog)
-        formatter._set_color(self.color)
+        formatter._set_color(self.color, file=file)
         return formatter
 
     # =====================
@@ -2734,12 +2736,14 @@ def _get_formatter(self):
     def print_usage(self, file=None):
         if file is None:
             file = _sys.stdout
-        self._print_message(self.format_usage(), file)
+        formatter = self._get_formatter(file=file)
+        self._print_message(self.format_usage(formatter=formatter), file)
 
     def print_help(self, file=None):
         if file is None:
             file = _sys.stdout
-        self._print_message(self.format_help(), file)
+        formatter = self._get_formatter(file=file)
+        self._print_message(self.format_help(formatter=formatter), file)
 
     def _print_message(self, message, file=None):
         if message:

Lib/test/test_argparse.py

@@ -7370,6 +7370,40 @@ def test_subparser_prog_is_stored_without_color(self):
         help_text = demo_parser.format_help()
         self.assertNotIn('\x1b[', help_text)
 
+    def test_print_help_uses_target_file_for_color_decision(self):
+        parser = argparse.ArgumentParser(prog='PROG', color=True)
+        parser.add_argument('--opt')
+        output = io.StringIO()
+        calls = []
+
+        def fake_can_colorize(*, file=None):
+            calls.append(file)
+            return file is None
+
+        with swap_attr(_colorize, 'can_colorize', fake_can_colorize):
+            parser.print_help(file=output)
+
+        self.assertIs(calls[-1], output)
+        self.assertIn(output, calls)
+        self.assertNotIn('\x1b[', output.getvalue())
+
+    def test_print_usage_uses_target_file_for_color_decision(self):
+        parser = argparse.ArgumentParser(prog='PROG', color=True)
+        parser.add_argument('--opt')
+        output = io.StringIO()
+        calls = []
+
+        def fake_can_colorize(*, file=None):
+            calls.append(file)
+            return file is None
+
+        with swap_attr(_colorize, 'can_colorize', fake_can_colorize):
+            parser.print_usage(file=output)
+
+        self.assertIs(calls[-1], output)
+        self.assertIn(output, calls)
+        self.assertNotIn('\x1b[', output.getvalue())
+
 
 class TestModule(unittest.TestCase):
     def test_deprecated__version__(self):

Misc/NEWS.d/next/Library/2025-10-23-06-38-35.gh-issue-139946.HZa5hu.rst

@@ -0,0 +1 @@
+Distinguish stdout and stderr when colorizing output in argparse module.