Fix handling empty values of NO_COLOR and FORCE_COLOR

AUTHORS

@@ -266,6 +266,7 @@ Michael Goerz
 Michael Krebs
 Michael Seifert
 Michal Wajszczuk
+Michał Górny
 Michał Zięba
 Mickey Pashov
 Mihai Capotă

changelog/11712.bugfix.rst

@@ -0,0 +1 @@
+Fixed handling ``NO_COLOR`` and ``FORCE_COLOR`` to ignore an empty value.

doc/en/reference/reference.rst

@@ -1146,13 +1146,13 @@ When set to ``0``, pytest will not use color.
 
 .. envvar:: NO_COLOR
 
-When set (regardless of value), pytest will not use color in terminal output.
+When set to a non-empty string (regardless of value), pytest will not use color in terminal output.
 ``PY_COLORS`` takes precedence over ``NO_COLOR``, which takes precedence over ``FORCE_COLOR``.
 See `no-color.org <https://no-color.org/>`__ for other libraries supporting this community standard.
 
 .. envvar:: FORCE_COLOR
 
-When set (regardless of value), pytest will use color in terminal output.
+When set to a non-empty string (regardless of value), pytest will use color in terminal output.
 ``PY_COLORS`` and ``NO_COLOR`` take precedence over ``FORCE_COLOR``.
 
 Exceptions

src/_pytest/_io/terminalwriter.py

@@ -29,9 +29,9 @@ def should_do_markup(file: TextIO) -> bool:
         return True
     if os.environ.get("PY_COLORS") == "0":
         return False
-    if "NO_COLOR" in os.environ:
+    if os.environ.get("NO_COLOR"):
         return False
-    if "FORCE_COLOR" in os.environ:
+    if os.environ.get("FORCE_COLOR"):
         return True
     return (
         hasattr(file, "isatty") and file.isatty() and os.environ.get("TERM") != "dumb"

testing/io/test_terminalwriter.py

@@ -5,6 +5,7 @@
 import sys
 from pathlib import Path
 from typing import Generator
+from typing import Optional
 from unittest import mock
 
 import pytest
@@ -164,53 +165,67 @@ def test_attr_hasmarkup() -> None:
     assert "\x1b[0m" in s
 
 
-def assert_color_set():
+def assert_color(expected: bool, default: Optional[bool] = None) -> None:
     file = io.StringIO()
-    tw = terminalwriter.TerminalWriter(file)
-    assert tw.hasmarkup
+    if default is None:
+        default = not expected
+    file.isatty = lambda: default  # type: ignore
+    tw = terminalwriter.TerminalWriter(file=file)
+    assert tw.hasmarkup is expected
     tw.line("hello", bold=True)
     s = file.getvalue()
-    assert len(s) > len("hello\n")
-    assert "\x1b[1m" in s
-    assert "\x1b[0m" in s
-
-
-def assert_color_not_set():
-    f = io.StringIO()
-    f.isatty = lambda: True  # type: ignore
-    tw = terminalwriter.TerminalWriter(file=f)
-    assert not tw.hasmarkup
-    tw.line("hello", bold=True)
-    s = f.getvalue()
-    assert s == "hello\n"
+    if expected:
+        assert len(s) > len("hello\n")
+        assert "\x1b[1m" in s
+        assert "\x1b[0m" in s
+    else:
+        assert s == "hello\n"
 
 
 def test_should_do_markup_PY_COLORS_eq_1(monkeypatch: MonkeyPatch) -> None:
     monkeypatch.setitem(os.environ, "PY_COLORS", "1")
-    assert_color_set()
+    assert_color(True)
 
 
 def test_should_not_do_markup_PY_COLORS_eq_0(monkeypatch: MonkeyPatch) -> None:
     monkeypatch.setitem(os.environ, "PY_COLORS", "0")
-    assert_color_not_set()
+    assert_color(False)
 
 
 def test_should_not_do_markup_NO_COLOR(monkeypatch: MonkeyPatch) -> None:
     monkeypatch.setitem(os.environ, "NO_COLOR", "1")
-    assert_color_not_set()
+    assert_color(False)
 
 
 def test_should_do_markup_FORCE_COLOR(monkeypatch: MonkeyPatch) -> None:
     monkeypatch.setitem(os.environ, "FORCE_COLOR", "1")
-    assert_color_set()
+    assert_color(True)
 
 
-def test_should_not_do_markup_NO_COLOR_and_FORCE_COLOR(
+@pytest.mark.parametrize(
+    ["NO_COLOR", "FORCE_COLOR", "expected"],
+    [
+        ("1", "1", False),
+        ("", "1", True),
+        ("1", "", False),
+    ],
+)
+def test_NO_COLOR_and_FORCE_COLOR(
     monkeypatch: MonkeyPatch,
+    NO_COLOR: str,
+    FORCE_COLOR: str,
+    expected: bool,
 ) -> None:
-    monkeypatch.setitem(os.environ, "NO_COLOR", "1")
-    monkeypatch.setitem(os.environ, "FORCE_COLOR", "1")
-    assert_color_not_set()
+    monkeypatch.setitem(os.environ, "NO_COLOR", NO_COLOR)
+    monkeypatch.setitem(os.environ, "FORCE_COLOR", FORCE_COLOR)
+    assert_color(expected)
+
+
+def test_empty_NO_COLOR_and_FORCE_COLOR_ignored(monkeypatch: MonkeyPatch) -> None:
+    monkeypatch.setitem(os.environ, "NO_COLOR", "")
+    monkeypatch.setitem(os.environ, "FORCE_COLOR", "")
+    assert_color(True, True)
+    assert_color(False, False)
 
 
 class TestTerminalWriterLineWidth: