Adds ability to configure stderr output color

docs/changelog/3426.misc.rst

@@ -0,0 +1,2 @@
+Adds ability to configure the stderr color for output received from external
+commands.

src/tox/config/cli/parser.py

@@ -11,6 +11,8 @@
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, Callable, Dict, List, Literal, Optional, Sequence, Tuple, Type, TypeVar, cast
 
+from colorama import Fore
+
 from tox.config.loader.str_convert import StrConvert
 from tox.plugin import NAME
 from tox.util.ci import is_ci
@@ -366,6 +368,12 @@ def add_color_flags(parser: ArgumentParser) -> None:
         choices=["yes", "no"],
         help="should output be enriched with colors, default is yes unless TERM=dumb or NO_COLOR is defined.",
     )
+    parser.add_argument(
+        "--stderr-color",
+        default="RED",
+        choices=[*Fore.__dict__.keys()],
+        help="color for stderr output, use RESET for terminal defaults.",
+    )
 
 
 def add_exit_and_dump_after(parser: ArgumentParser) -> None:

src/tox/execute/api.py

@@ -122,11 +122,18 @@ def call(
         env: ToxEnv,
     ) -> Iterator[ExecuteStatus]:
         start = time.monotonic()
+        stderr_color = None
+        if self._colored:
+            try:
+                cfg_color = env.conf._conf.options.stderr_color  # noqa: SLF001
+                stderr_color = Fore.__dict__[cfg_color]
+            except (AttributeError, KeyError, TypeError):  # many tests have a mocked 'env'
+                stderr_color = Fore.RED
         try:
             # collector is what forwards the content from the file streams to the standard streams
             out, err = out_err[0].buffer, out_err[1].buffer
             out_sync = SyncWrite(out.name, out if show else None)
-            err_sync = SyncWrite(err.name, err if show else None, Fore.RED if self._colored else None)
+            err_sync = SyncWrite(err.name, err if show else None, stderr_color)
             with out_sync, err_sync:
                 instance = self.build_instance(request, self._option_class(env), out_sync, err_sync)
                 with instance as status:
@@ -265,7 +272,7 @@ def _assert_fail(self) -> NoReturn:
                 if not self.out.endswith("\n"):
                     sys.stdout.write("\n")
             if self.err:
-                sys.stderr.write(Fore.RED)
+                sys.stderr.write(Fore.GREEN)
                 sys.stderr.write(self.err)
                 sys.stderr.write(Fore.RESET)
                 if not self.err.endswith("\n"):

tests/config/cli/test_cli_env_var.py

@@ -31,6 +31,7 @@ def test_verbose_no_test() -> None:
         "verbose": 4,
         "quiet": 0,
         "colored": "no",
+        "stderr_color": "RED",
         "work_dir": None,
         "root_dir": None,
         "config_file": None,
@@ -90,6 +91,7 @@ def test_env_var_exhaustive_parallel_values(
     assert vars(options.parsed) == {
         "always_copy": False,
         "colored": "no",
+        "stderr_color": "RED",
         "command": "legacy",
         "default_runner": "virtualenv",
         "develop": False,

tests/config/cli/test_cli_ini.py

@@ -29,6 +29,7 @@
 def default_options() -> dict[str, Any]:
     return {
         "colored": "no",
+        "stderr_color": "RED",
         "command": "r",
         "default_runner": "virtualenv",
         "develop": False,
@@ -200,6 +201,7 @@ def test_ini_exhaustive_parallel_values(core_handlers: dict[str, Callable[[State
     options = get_options("p")
     assert vars(options.parsed) == {
         "colored": "yes",
+        "stderr_color": "RED",
         "command": "p",
         "default_runner": "virtualenv",
         "develop": False,

tests/execute/local_subprocess/test_local_subprocess.py

@@ -48,20 +48,30 @@ def read_out_err(self) -> tuple[str, str]:
 @pytest.mark.parametrize("color", [True, False], ids=["color", "no_color"])
 @pytest.mark.parametrize(("out", "err"), [("out", "err"), ("", "")], ids=["simple", "nothing"])
 @pytest.mark.parametrize("show", [True, False], ids=["show", "no_show"])
+@pytest.mark.parametrize(
+    "stderr_color",
+    ["RED", "YELLOW", "RESET"],
+    ids=["stderr_color_default", "stderr_color_yellow", "stderr_color_reset"],
+)
 def test_local_execute_basic_pass(  # noqa: PLR0913
     caplog: LogCaptureFixture,
     os_env: dict[str, str],
     out: str,
     err: str,
     show: bool,
     color: bool,
+    stderr_color: str,
 ) -> None:
     caplog.set_level(logging.NOTSET)
     executor = LocalSubProcessExecutor(colored=color)
+
+    tox_env = MagicMock()
+    tox_env.conf._conf.options.stderr_color = stderr_color  # noqa: SLF001
     code = f"import sys; print({out!r}, end=''); print({err!r}, end='', file=sys.stderr)"
     request = ExecuteRequest(cmd=[sys.executable, "-c", code], cwd=Path(), env=os_env, stdin=StdinSource.OFF, run_id="")
     out_err = FakeOutErr()
-    with executor.call(request, show=show, out_err=out_err.out_err, env=MagicMock()) as status:
+
+    with executor.call(request, show=show, out_err=out_err.out_err, env=tox_env) as status:
         while status.exit_code is None:  # pragma: no branch
             status.wait()
     assert status.out == out.encode()
@@ -77,7 +87,7 @@ def test_local_execute_basic_pass(  # noqa: PLR0913
     out_got, err_got = out_err.read_out_err()
     if show:
         assert out_got == out
-        expected = (f"{Fore.RED}{err}{Fore.RESET}" if color else err) if err else ""
+        expected = f"{Fore.__dict__[stderr_color]}{err}{Fore.RESET}" if color and err else err
         assert err_got == expected
     else:
         assert not out_got
@@ -226,7 +236,9 @@ def test_local_execute_basic_fail(capsys: CaptureFixture, caplog: LogCaptureFixt
 
     out, err = capsys.readouterr()
     assert out == "out\n"
-    expected = f"{Fore.RED}err{Fore.RESET}\n"
+    # Because this is a command that is expected to fail, we expect it to be
+    # colored using green (_assert_fail) and not the default red.
+    expected = f"{Fore.GREEN}err{Fore.RESET}\n"
     assert err == expected
 
     assert len(caplog.records) == 1