monochrome tests

src/textual/_styles_cache.py

@@ -7,6 +7,7 @@ from rich.segment import Segment
 from rich.style import Style
 
 from ._border import get_box, render_row
+from ._filter import LineFilter
 from ._opacity import _apply_opacity
 from ._segment_tools import line_crop, line_pad, line_trim
 from ._types import Lines
@@ -134,6 +135,7 @@ class StylesCache:
             content_size=widget.content_region.size,
             padding=styles.padding,
             crop=crop,
+            filter=widget.app._filter,
         )
         if widget.auto_links:
             _style_links = style_links
@@ -163,6 +165,7 @@ class StylesCache:
         content_size: Size | None = None,
         padding: Spacing | None = None,
         crop: Region | None = None,
+        filter: LineFilter | None = None,
     ) -> Lines:
         """Render a widget content plus CSS styles.
 
@@ -212,6 +215,8 @@ class StylesCache:
                 self._cache[y] = line
             else:
                 line = self._cache[y]
+            if filter:
+                line = filter.filter(line)
             add_line(line)
         self._dirty_lines.difference_update(crop.line_range)
 

src/textual/app.py

@@ -35,6 +35,7 @@ from ._animator import Animator, DEFAULT_EASING, Animatable, EasingFunction
 from ._callback import invoke
 from ._context import active_app
 from ._event_broker import NoHandler, extract_handler_actions
+from ._filter import LineFilter, Monochrome
 from .binding import Bindings, NoBinding
 from .css.query import NoMatchingNodesError
 from .css.stylesheet import Stylesheet
@@ -163,12 +164,18 @@ class App(Generic[ReturnType], DOMNode):
         super().__init__()
         self.features: frozenset[FeatureFlag] = parse_features(os.getenv("TEXTUAL", ""))
 
+        self._filter: LineFilter | None = None
+        environ = dict(os.environ)
+        no_color = environ.pop("NO_COLOR", None)
+        if no_color is not None:
+            self._filter = Monochrome()
         self.console = Console(
             file=(_NullFile() if self.is_headless else sys.__stdout__),
             markup=False,
             highlight=False,
             emoji=False,
             legacy_windows=False,
+            _environ=environ,
         )
         self.error_console = Console(markup=False, stderr=True)
         self.driver_class = driver_class or self.get_driver_class()

src/textual/color.py

@@ -311,6 +311,17 @@ class Color(NamedTuple):
         r, g, b, a = self
         return f"rgb({r},{g},{b})" if a == 1 else f"rgba({r},{g},{b},{a})"
 
+    @property
+    def monochrome(self) -> Color:
+        """Get a monochrome version of this color.
+
+        Returns:
+            Color: A new monochrome color.
+        """
+        r, g, b, a = self
+        gray = round(r * 0.2126 + g * 0.7152 + b * 0.0722)
+        return Color(gray, gray, gray, a)
+
     def __rich_repr__(self) -> rich.repr.Result:
         r, g, b, a = self
         yield r

tests/test_color.py

@@ -31,6 +31,13 @@ def test_css():
     assert Color(10, 20, 30, 0.5).css == "rgba(10,20,30,0.5)"
 
 
+def test_monochrome():
+    assert Color(10, 20, 30).monochrome == Color(19, 19, 19)
+    assert Color(10, 20, 30, 0.5).monochrome == Color(19, 19, 19, 0.5)
+    assert Color(255, 255, 255).monochrome == Color(255, 255, 255)
+    assert Color(0, 0, 0).monochrome == Color(0, 0, 0)
+
+
 def test_rgb():
     assert Color(10, 20, 30, 0.55).rgb == (10, 20, 30)