gh-128595: Default to stdout isatty for colour detection instead of stderr

Lib/_colorize.py

@@ -26,14 +26,20 @@ class ANSIColors:
         setattr(NoColors, attr, "")
 
 
-def get_colors(colorize: bool = False) -> ANSIColors:
-    if colorize or can_colorize():
+def get_colors(colorize: bool = False, *, file=None) -> ANSIColors:
+    if file is None:
+        file = sys.stdout
+
+    if colorize or can_colorize(file=file):
         return ANSIColors()
     else:
         return NoColors
 
 
-def can_colorize() -> bool:
+def can_colorize(*, file=None) -> bool:
+    if file is None:
+        file = sys.stdout
+
     if not sys.flags.ignore_environment:
         if os.environ.get("PYTHON_COLORS") == "0":
             return False
@@ -49,7 +55,7 @@ def can_colorize() -> bool:
         if os.environ.get("TERM") == "dumb":
             return False
 
-    if not hasattr(sys.stderr, "fileno"):
+    if not hasattr(file, "fileno"):
         return False
 
     if sys.platform == "win32":
@@ -62,6 +68,6 @@ def can_colorize() -> bool:
             return False
 
     try:
-        return os.isatty(sys.stderr.fileno())
+        return os.isatty(file.fileno())
     except io.UnsupportedOperation:
-        return sys.stderr.isatty()
+        return sys.stdout.isatty()

Lib/doctest.py

@@ -1558,7 +1558,7 @@ def out(s):
         save_displayhook = sys.displayhook
         sys.displayhook = sys.__displayhook__
         saved_can_colorize = _colorize.can_colorize
-        _colorize.can_colorize = lambda: False
+        _colorize.can_colorize = lambda *args, **kwargs: False
         color_variables = {"PYTHON_COLORS": None, "FORCE_COLOR": None}
         for key in color_variables:
             color_variables[key] = os.environ.pop(key, None)

Lib/test/libregrtest/single.py

@@ -162,7 +162,7 @@ def test_func():
 def _runtest_env_changed_exc(result: TestResult, runtests: RunTests,
                              display_failure: bool = True) -> None:
     # Handle exceptions, detect environment changes.
-    ansi = get_colors()
+    ansi = get_colors(file=sys.stderr)
     red, reset, yellow = ansi.RED, ansi.RESET, ansi.YELLOW
 
     # Reset the environment_altered flag to detect if a test altered
@@ -303,7 +303,7 @@ def run_single_test(test_name: TestName, runtests: RunTests) -> TestResult:
     If runtests.use_junit, xml_data is a list containing each generated
     testsuite element.
     """
-    ansi = get_colors()
+    ansi = get_colors(file=sys.stderr)
     red, reset, yellow = ansi.BOLD_RED, ansi.RESET, ansi.YELLOW
 
     start_time = time.perf_counter()

Lib/test/support/__init__.py

@@ -2839,7 +2839,7 @@ def no_color():
     from .os_helper import EnvironmentVarGuard
 
     with (
-        swap_attr(_colorize, "can_colorize", lambda: False),
+        swap_attr(_colorize, "can_colorize", lambda file=None: False),
         EnvironmentVarGuard() as env,
     ):
         for var in {"FORCE_COLOR", "NO_COLOR", "PYTHON_COLORS"}:

Lib/traceback.py

@@ -135,7 +135,7 @@ def print_exception(exc, /, value=_sentinel, tb=_sentinel, limit=None, \
 
 def _print_exception_bltin(exc, /):
     file = sys.stderr if sys.stderr is not None else sys.__stderr__
-    colorize = _colorize.can_colorize()
+    colorize = _colorize.can_colorize(file=file)
     return print_exception(exc, limit=BUILTIN_EXCEPTION_LIMIT, file=file, colorize=colorize)
 
 

Lib/unittest/result.py

@@ -191,7 +191,8 @@ def _exc_info_to_string(self, err, test):
             capture_locals=self.tb_locals, compact=True)
         from _colorize import can_colorize
 
-        msgLines = list(tb_e.format(colorize=can_colorize()))
+        file = self.stream if hasattr(self, "stream") else None
+        msgLines = list(tb_e.format(colorize=can_colorize(file=file)))
 
         if self.buffer:
             output = sys.stdout.getvalue()

Lib/unittest/runner.py

@@ -45,7 +45,8 @@ def __init__(self, stream, descriptions, verbosity, *, durations=None):
         self.showAll = verbosity > 1
         self.dots = verbosity == 1
         self.descriptions = descriptions
-        self._ansi = get_colors()
+        file = sys.stderr if stream == "<stderr>" else None
+        self._ansi = get_colors(file=file)
         self._newline = True
         self.durations = durations
 
@@ -286,7 +287,8 @@ def run(self, test):
             expected_fails, unexpected_successes, skipped = results
 
         infos = []
-        ansi = get_colors()
+        file = sys.stderr if self.stream == "<stderr>" else None
+        ansi = get_colors(file=file)
         bold_red = ansi.BOLD_RED
         green = ansi.GREEN
         red = ansi.RED

Misc/NEWS.d/next/Library/2025-01-07-21-48-32.gh-issue-128498.n6jtlW.rst

@@ -0,0 +1,2 @@
+Default to stdout isatty for colour detection instead of stderr. Patch by
+Hugo van Kemenade.