Take the FORCE_COLOR and NO_COLOR environment variables into account

doc/user_guide/usage/output.rst

@@ -21,6 +21,19 @@ a colorized report to stdout at the same time:
 
   --output-format=json:somefile.json,colorized
 
+Environment Variables
+''''''''''''''''''''''''''''
+The colorization of the output can also be controlled through environment
+variables. The precedence for determining output format is as follows:
+
+1. ``NO_COLOR``
+2. ``FORCE_COLOR``, or ``PY_COLORS``
+3. ``--output-format=...``
+
+Setting ``NO_COLOR`` (to any value) will disable colorized output, while
+``FORCE_COLOR`` or ``PY_COLORS`` (to any value) will enable it, overriding
+the ``--output-format`` option if specified.
+
 
 Custom message formats
 ''''''''''''''''''''''

doc/whatsnew/fragments/3995.feature

@@ -0,0 +1,5 @@
+Support for ``NO_COLOR`` and ``FORCE_COLOR`` (and ``PY_COLORS``, as an alias to ``FORCE_COLOR``) environment variables has been added.
+When running `pylint`, the reporter that reports to ``stdout`` will be modified according to the requested mode.
+The order is: ``NO_COLOR`` > ``FORCE_COLOR``, ``PY_COLORS`` > ``--output=...``.
+
+Closes #3995 (https://github.com/pylint-dev/pylint/issues/3995).

pylint/lint/pylinter.py

@@ -12,6 +12,7 @@
 import sys
 import tokenize
 import traceback
+import warnings
 from collections import defaultdict
 from collections.abc import Callable, Iterable, Iterator, Sequence
 from io import TextIOWrapper
@@ -48,13 +49,15 @@
     report_total_messages_stats,
 )
 from pylint.lint.utils import (
+    _is_env_set_and_non_empty,
     augmented_sys_path,
     get_fatal_error_message,
     prepare_crash_report,
 )
 from pylint.message import Message, MessageDefinition, MessageDefinitionStore
+from pylint.reporters import ReporterWarning
 from pylint.reporters.base_reporter import BaseReporter
-from pylint.reporters.text import TextReporter
+from pylint.reporters.text import ColorizedTextReporter, TextReporter
 from pylint.reporters.ureports import nodes as report_nodes
 from pylint.typing import (
     DirectoryNamespaceDict,
@@ -69,6 +72,14 @@
 
 MANAGER = astroid.MANAGER
 
+NO_COLOR = "NO_COLOR"
+FORCE_COLOR = "FORCE_COLOR"
+PY_COLORS = "PY_COLORS"
+
+WARN_FORCE_COLOR_SET = "FORCE_COLOR is set; ignoring `text` at stdout"
+WARN_NO_COLOR_SET = "NO_COLOR is set; ignoring `colorized` at stdout"
+WARN_BOTH_COLOR_SET = "Both NO_COLOR and FORCE_COLOR are set! (disabling colors)"
+
 
 class GetAstProtocol(Protocol):
     def __call__(
@@ -250,6 +261,68 @@
 }
 
 
+def _handle_force_color_no_color(reporter: list[reporters.BaseReporter]) -> None:
+    """
+    Check ``NO_COLOR``, ``FORCE_COLOR``, ``PY_COLOR`` and modify the reporter list
+    accordingly.
+
+    Rules are presented in this table:
+    +--------------+---------------+-----------------+------------------------------------------------------------+
+    | `NO_COLOR`   | `FORCE_COLOR` | `output-format` | Behavior                                                   |
+    +==============+===============+=================+============================================================+
+    | `bool: True` | `bool: True`  | colorized       | not colorized + warnings (override + inconsistent env var) |
+    | `bool: True` | `bool: True`  | /               | not colorized + warnings (inconsistent env var)            |
+    | unset        | `bool: True`  | colorized       | colorized                                                  |
+    | unset        | `bool: True`  | /               | colorized + warnings (override)                            |
+    | `bool: True` | unset         | colorized       | not colorized + warnings (override)                        |
+    | `bool: True` | unset         | /               | not colorized                                              |
+    | unset        | unset         | colorized       | colorized                                                  |
+    | unset        | unset         | /               | not colorized                                              |
+    +--------------+---------------+-----------------+------------------------------------------------------------+
+    """
+    no_color = _is_env_set_and_non_empty(NO_COLOR)
+    force_color = _is_env_set_and_non_empty(FORCE_COLOR) or _is_env_set_and_non_empty(
+        PY_COLORS
+    )
+
+    if no_color and force_color:
+        warnings.warn(
+            WARN_BOTH_COLOR_SET,
+            ReporterWarning,
+            stacklevel=2,
+        )
+        force_color = False
+
+    if no_color:
+        for idx, rep in enumerate(list(reporter)):
+            if not isinstance(rep, ColorizedTextReporter):
+                continue
+
+            if rep.out.buffer is sys.stdout.buffer:
+                warnings.warn(
+                    WARN_NO_COLOR_SET,
+                    ReporterWarning,
+                    stacklevel=2,
+                )
+                reporter.pop(idx)
+                reporter.append(TextReporter())
+
+    elif force_color:
+        for idx, rep in enumerate(list(reporter)):
+            # pylint: disable=unidiomatic-typecheck # Want explicit type check
+            if type(rep) is not TextReporter:
+                continue
+
+            if rep.out.buffer is sys.stdout.buffer:
+                warnings.warn(
+                    WARN_FORCE_COLOR_SET,
+                    ReporterWarning,
+                    stacklevel=2,
+                )
+                reporter.pop(idx)
+                reporter.append(ColorizedTextReporter())
+
+
 # pylint: disable=too-many-instance-attributes,too-many-public-methods
 class PyLinter(
     _ArgumentsManager,
@@ -435,6 +508,8 @@
             # Extend the lifetime of all opened output files
             close_output_files = stack.pop_all().close
 
+        _handle_force_color_no_color(sub_reporters)
+
         if len(sub_reporters) > 1 or output_files:
             self.set_reporter(
                 reporters.MultiReporter(

pylint/lint/utils.py

@@ -5,6 +5,7 @@
 from __future__ import annotations
 
 import contextlib
+import os
 import platform
 import sys
 import traceback
@@ -133,3 +134,21 @@ def augmented_sys_path(additional_paths: Sequence[str]) -> Iterator[None]:
         yield
     finally:
         sys.path[:] = original
+
+
+def _is_relative_to(self: Path, *other: Path) -> bool:
+    """Checks if self is relative to other.
+
+    Backport of pathlib.Path.is_relative_to for Python <3.9
+    TODO: py39: Remove this backport and use stdlib function.
+    """
+    try:
+        self.relative_to(*other)
+        return True
+    except ValueError:
+        return False
+
+
+def _is_env_set_and_non_empty(env_var: str) -> bool:
+    """Checks if env_var is set and non-empty."""
+    return bool(os.environ.get(env_var))

pylint/reporters/__init__.py

@@ -19,6 +19,10 @@
     from pylint.lint.pylinter import PyLinter
 
 
+class ReporterWarning(Warning):
+    """Warning class for reporters."""
+
+
 def initialize(linter: PyLinter) -> None:
     """Initialize linter with reporters in this package."""
     utils.register_plugins(linter, __path__[0])

tests/lint/test_pylinter.py

@@ -2,17 +2,35 @@
 # For details: https://github.com/pylint-dev/pylint/blob/main/LICENSE
 # Copyright (c) https://github.com/pylint-dev/pylint/blob/main/CONTRIBUTORS.txt
 
+from __future__ import annotations
+
+import io
 import os
+import sys
+import warnings
 from pathlib import Path
 from typing import Any, NoReturn
 from unittest import mock
 from unittest.mock import patch
 
+import pytest
 from pytest import CaptureFixture
 
-from pylint.lint.pylinter import MANAGER, PyLinter
+from pylint.lint.pylinter import (
+    FORCE_COLOR,
+    NO_COLOR,
+    PY_COLORS,
+    WARN_BOTH_COLOR_SET,
+    PyLinter,
+    _handle_force_color_no_color,
+)
+from pylint.reporters.text import ColorizedTextReporter, TextReporter
 from pylint.utils import FileState
 
+COLORIZED_REPORTERS = "colorized_reporters"
+TEXT_REPORTERS = "text_reporters"
+STDOUT_TEXT = "stdout"
+
 
 def raise_exception(*args: Any, **kwargs: Any) -> NoReturn:
     raise ValueError
@@ -50,21 +68,143 @@
         assert any(m.symbol == "astroid-error" for m in linter.reporter.messages)
 
 
+def pytest_generate_tests(metafunc: pytest.Metafunc) -> None:
+    if metafunc.function.__name__ != test_handle_force_color_no_color.__name__:
+        return
+
+    if (
+        TEXT_REPORTERS not in metafunc.fixturenames
+        or COLORIZED_REPORTERS not in metafunc.fixturenames
+    ):
+        warnings.warn(
+            f"Missing fixture {TEXT_REPORTERS} or {COLORIZED_REPORTERS} in"
+            f" {test_handle_force_color_no_color.function.__name__}??",
+            stacklevel=2,
+        )
+        return
+
+    parameters = []
+
+    reporter_combinations = [
+        ("file", STDOUT_TEXT),
+        ("file",),
+        (STDOUT_TEXT,),
+        ("",),
+    ]
+
+    for tr in list(reporter_combinations):
+        for cr in list(reporter_combinations):
+            tr = tuple(t for t in tr if t)
+            cr = tuple(t for t in cr if t)
+
+            total_reporters = len(tr) + len(cr)
+            unique_reporters = len(set(tr + cr))
+
+            if total_reporters == 0:
+                continue
+
+            if unique_reporters != total_reporters:
+                continue
+
+            parameters.append((tuple(tr), tuple(cr)))
+
+    metafunc.parametrize(
+        f"{TEXT_REPORTERS}, {COLORIZED_REPORTERS}", parameters, ids=repr
+    )
+
+
+@pytest.mark.parametrize("no_color", [True, False], ids=lambda no_color: f"{no_color=}")
+@pytest.mark.parametrize(
+    "py_colors", [True, False], ids=lambda py_colors: f"{py_colors=}"
+)
+@pytest.mark.parametrize(
+    "force_color", [True, False], ids=lambda force_color: f"{force_color=}"
+)
+def test_handle_force_color_no_color(
+    monkeypatch: pytest.MonkeyPatch,
+    recwarn: pytest.WarningsRecorder,
+    no_color: bool,
+    py_colors: bool,
+    force_color: bool,
+    text_reporters: tuple[str],
+    colorized_reporters: tuple[str],
+) -> None:
+    monkeypatch.setenv(NO_COLOR, "1" if no_color else "")
+    monkeypatch.setenv(FORCE_COLOR, "1" if force_color else "")
+    monkeypatch.setenv(PY_COLORS, "1" if py_colors else "")
+
+    force_color = force_color or py_colors
+
+    if STDOUT_TEXT in text_reporters or STDOUT_TEXT in colorized_reporters:
+        monkeypatch.setattr(sys, STDOUT_TEXT, io.TextIOWrapper(io.BytesIO()))
+
+    reporters = []
+    for reporter, group in (
+        (TextReporter, text_reporters),
+        (ColorizedTextReporter, colorized_reporters),
+    ):
+        for name in group:
+            if name == STDOUT_TEXT:
+                reporters.append(reporter())
+            if name == "file":
+                reporters.append(reporter(io.TextIOWrapper(io.BytesIO())))
+
+    _handle_force_color_no_color(reporters)
+
+    if no_color and force_color:
+        # Both NO_COLOR and FORCE_COLOR are set; expecting a warning.
+        both_color_warning = [
+            idx
+            for idx, w in enumerate(recwarn.list)
+            if WARN_BOTH_COLOR_SET in str(w.message)
+        ]
+        assert len(both_color_warning) == 1
+        recwarn.list.pop(both_color_warning[0])
+
+    if no_color:
+        # No ColorizedTextReporter expected to be connected to stdout.
+        assert all(
+            not isinstance(rep, ColorizedTextReporter)
+            for rep in reporters
+            if rep.out.buffer is sys.stdout.buffer
+        )
+
+        if STDOUT_TEXT in colorized_reporters:
+            assert len(recwarn.list) == 1  # expect a warning for overriding stdout
+        else:
+            assert len(recwarn.list) == 0  # no warning expected
+    elif force_color:
+        # No TextReporter expected to be connected to stdout.
+        # pylint: disable=unidiomatic-typecheck # Want explicit type check.
+        assert all(
+            type(rep) is not TextReporter
+            for rep in reporters
+            if rep.out.buffer is sys.stdout.buffer
+        )
+
+        if STDOUT_TEXT in text_reporters:
+            assert len(recwarn.list) == 1  # expect a warning for overriding stdout
+        else:
+            assert len(recwarn.list) == 0  # no warning expected
+    else:
+        assert len(recwarn.list) == 0  # no warning expected
+
+
 def test_open_pylinter_denied_modules(linter: PyLinter) -> None:
     """Test PyLinter open() adds ignored modules to Astroid manager deny list."""
     MANAGER.module_denylist = {"mod1"}
     try:
         linter.config.ignored_modules = ["mod2", "mod3"]
         linter.open()
         assert MANAGER.module_denylist == {"mod1", "mod2", "mod3"}
     finally:
         MANAGER.module_denylist = set()
 
 
 def test_open_pylinter_prefer_stubs(linter: PyLinter) -> None:
     try:
         linter.config.prefer_stubs = True
         linter.open()
         assert MANAGER.prefer_stubs
     finally:
         MANAGER.prefer_stubs = False