Streamline testing for FORCE_COLOR and NO_COLOR

Streamline the tests for FORCE_COLOR and NO_COLOR variables, and cover
all possible cases (unset, set to empty, set to "1").  Combine the two
assert functions into one taking boolean parameters.  Mock file.isatty
in all circumstances to ensure that the environment variables take
precedence over the fallback value resulting from isatty check (or that
the fallback is actually used, in the case of both FORCE_COLOR
and NO_COLOR being unset).

testing/io/test_terminalwriter.py

@@ -5,6 +5,7 @@ import shutil
 import sys
 from pathlib import Path
 from typing import Generator
+from typing import Optional
 from unittest import mock
 
 import pytest
@@ -164,59 +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_ignored(monkeypatch: MonkeyPatch) -> None:
+def test_empty_NO_COLOR_and_FORCE_COLOR_ignored(monkeypatch: MonkeyPatch) -> None:
     monkeypatch.setitem(os.environ, "NO_COLOR", "")
-    monkeypatch.setitem(os.environ, "FORCE_COLOR", "1")
-    assert_color_set()
+    monkeypatch.setitem(os.environ, "FORCE_COLOR", "")
+    assert_color(True, True)
+    assert_color(False, False)
 
 
 class TestTerminalWriterLineWidth: