BUG: Bg gradient map text color fix (#39889)

Author: attack68

doc/source/whatsnew/v1.3.0.rst

@@ -480,6 +480,7 @@ Other
 - Bug in :class:`Styler` where ``subset`` arg in methods raised an error for some valid multiindex slices (:issue:`33562`)
 - :class:`Styler` rendered HTML output minor alterations to support w3 good code standard (:issue:`39626`)
 - Bug in :class:`Styler` where rendered HTML was missing a column class identifier for certain header cells (:issue:`39716`)
+- Bug in :meth:`Styler.background_gradient` where text-color was not determined correctly (:issue:`39888`)
 - Bug in :meth:`DataFrame.equals`, :meth:`Series.equals`, :meth:`Index.equals` with object-dtype containing ``np.datetime64("NaT")`` or ``np.timedelta64("NaT")`` (:issue:`39650`)
 
 

pandas/io/formats/style.py

@@ -1428,7 +1428,7 @@ def relative_luminance(rgba) -> float:
                     The relative luminance as a value from 0 to 1
                 """
                 r, g, b = (
-                    x / 12.92 if x <= 0.03928 else ((x + 0.055) / 1.055 ** 2.4)
+                    x / 12.92 if x <= 0.04045 else ((x + 0.055) / 1.055) ** 2.4
                     for x in rgba[:3]
                 )
                 return 0.2126 * r + 0.7152 * g + 0.0722 * b

pandas/tests/io/formats/test_style.py

@@ -1943,28 +1943,36 @@ def test_background_gradient(self):
         assert result[(1, 0)] == [("background-color", "#fff7fb"), ("color", "#000000")]
 
     @pytest.mark.parametrize(
-        "c_map,expected",
+        "cmap, expected",
         [
             (
-                None,
+                "PuBu",
                 {
-                    (0, 0): [("background-color", "#440154"), ("color", "#f1f1f1")],
-                    (1, 0): [("background-color", "#fde725"), ("color", "#000000")],
+                    (4, 5): [("background-color", "#86b0d3"), ("color", "#000000")],
+                    (4, 6): [("background-color", "#83afd3"), ("color", "#f1f1f1")],
                 },
             ),
             (
                 "YlOrRd",
                 {
-                    (0, 0): [("background-color", "#ffffcc"), ("color", "#000000")],
-                    (1, 0): [("background-color", "#800026"), ("color", "#f1f1f1")],
+                    (4, 8): [("background-color", "#fd913e"), ("color", "#000000")],
+                    (4, 9): [("background-color", "#fd8f3d"), ("color", "#f1f1f1")],
+                },
+            ),
+            (
+                None,
+                {
+                    (7, 0): [("background-color", "#48c16e"), ("color", "#f1f1f1")],
+                    (7, 1): [("background-color", "#4cc26c"), ("color", "#000000")],
                 },
             ),
         ],
     )
-    def test_text_color_threshold(self, c_map, expected):
-        df = DataFrame([1, 2], columns=["A"])
-        result = df.style.background_gradient(cmap=c_map)._compute().ctx
-        assert result == expected
+    def test_text_color_threshold(self, cmap, expected):
+        df = DataFrame(np.arange(100).reshape(10, 10))
+        result = df.style.background_gradient(cmap=cmap, axis=None)._compute().ctx
+        for k in expected.keys():
+            assert result[k] == expected[k]
 
     @pytest.mark.parametrize("text_color_threshold", [1.1, "1", -1, [2, 2]])
     def test_text_color_threshold_raises(self, text_color_threshold):