ENH: Add support for tablewise application of style.background_gradient with axis=None

doc/source/whatsnew/v0.24.0.txt

@@ -425,6 +425,7 @@ Other
 
 - :meth: `~pandas.io.formats.style.Styler.background_gradient` now takes a ``text_color_threshold`` parameter to automatically lighten the text color based on the luminance of the background color. This improves readability with dark background colors without the need to limit the background colormap range. (:issue:`21258`)
 - Require at least 0.28.2 version of ``cython`` to support read-only memoryviews (:issue:`21688`)
+- :meth: `~pandas.io.formats.style.Styler.background_gradient` now also supports tablewise application (in addition to rowwise and columnwise) with ``axis=None`` (:issue:`15204`)
 -
 -
 -

pandas/io/formats/style.py

@@ -913,21 +913,22 @@ def background_gradient(self, cmap='PuBu', low=0, high=0, axis=0,
     def _background_gradient(s, cmap='PuBu', low=0, high=0,
                              text_color_threshold=0.408):
         """Color background in a range according to the data."""
+        if (not isinstance(text_color_threshold, (float, int)) or
+                not 0 <= text_color_threshold <= 1):
+            msg = "`text_color_threshold` must be a value from 0 to 1."
+            raise ValueError(msg)
+
         with _mpl(Styler.background_gradient) as (plt, colors):
-            rng = s.max() - s.min()
+            smin = s.values.min()
+            smax = s.values.max()
+            rng = smax - smin
             # extend lower / upper bounds, compresses color range
-            norm = colors.Normalize(s.min() - (rng * low),
-                                    s.max() + (rng * high))
-            # matplotlib modifies inplace?
+            norm = colors.Normalize(smin - (rng * low), smax + (rng * high))
+            # matplotlib colors.Normalize modifies inplace?
             # https://github.com/matplotlib/matplotlib/issues/5427
-            normed = norm(s.values)
-            c = [colors.rgb2hex(x) for x in plt.cm.get_cmap(cmap)(normed)]
-            if (not isinstance(text_color_threshold, (float, int)) or
-                    not 0 <= text_color_threshold <= 1):
-                msg = "`text_color_threshold` must be a value from 0 to 1."
-                raise ValueError(msg)
+            rgbas = plt.cm.get_cmap(cmap)(norm(s.values))
 
-            def relative_luminance(color):
+            def relative_luminance(rgba):
                 """
                 Calculate relative luminance of a color.
 
@@ -936,25 +937,33 @@ def relative_luminance(color):
 
                 Parameters
                 ----------
-                color : matplotlib color
-                    Hex code, rgb-tuple, or HTML color name.
+                color : rgb or rgba tuple
 
                 Returns
                 -------
                 float
                     The relative luminance as a value from 0 to 1
                 """
-                rgb = colors.colorConverter.to_rgba_array(color)[:, :3]
-                rgb = np.where(rgb <= .03928, rgb / 12.92,
-                               ((rgb + .055) / 1.055) ** 2.4)
-                lum = rgb.dot([.2126, .7152, .0722])
-                return lum.item()
-
-            text_colors = ['#f1f1f1' if relative_luminance(x) <
-                           text_color_threshold else '#000000' for x in c]
-
-            return ['background-color: {color};color: {tc}'.format(
-                    color=color, tc=tc) for color, tc in zip(c, text_colors)]
+                r, g, b = (
+                    x / 12.92 if x <= 0.03928 else ((x + 0.055) / 1.055 ** 2.4)
+                    for x in rgba[:3]
+                )
+                return 0.2126 * r + 0.7152 * g + 0.0722 * b
+
+            def css(rgba):
+                dark = relative_luminance(rgba) < text_color_threshold
+                text_color = '#f1f1f1' if dark else '#000000'
+                return 'background-color: {b};color: {c};'.format(
+                    b=colors.rgb2hex(rgba), c=text_color
+                )
+
+            if s.ndim == 1:
+                return [css(rgba) for rgba in rgbas]
+            else:
+                return pd.DataFrame(
+                    [[css(rgba) for rgba in row] for row in rgbas],
+                    index=s.index, columns=s.columns
+                )
 
     def set_properties(self, subset=None, **kwargs):
         """

pandas/tests/io/formats/test_style.py

@@ -1056,6 +1056,34 @@ def test_text_color_threshold_raises(self, text_color_threshold):
             df.style.background_gradient(
                 text_color_threshold=text_color_threshold)._compute()
 
+    @td.skip_if_no_mpl
+    def test_background_gradient_axis(self):
+        df = pd.DataFrame([[1, 2], [2, 4]], columns=['A', 'B'])
+
+        low = ['background-color: #f7fbff', 'color: #000000']
+        high = ['background-color: #08306b', 'color: #f1f1f1']
+        mid = ['background-color: #abd0e6', 'color: #000000']
+        result = df.style.background_gradient(cmap='Blues',
+                                              axis=0)._compute().ctx
+        assert result[(0, 0)] == low
+        assert result[(0, 1)] == low
+        assert result[(1, 0)] == high
+        assert result[(1, 1)] == high
+
+        result = df.style.background_gradient(cmap='Blues',
+                                              axis=1)._compute().ctx
+        assert result[(0, 0)] == low
+        assert result[(0, 1)] == high
+        assert result[(1, 0)] == low
+        assert result[(1, 1)] == high
+
+        result = df.style.background_gradient(cmap='Blues',
+                                              axis=None)._compute().ctx
+        assert result[(0, 0)] == low
+        assert result[(0, 1)] == mid
+        assert result[(1, 0)] == mid
+        assert result[(1, 1)] == high
+
 
 def test_block_names():
     # catch accidental removal of a block