Fixing _get_standard_colors to honor num_colors arguments (#20585)

doc/source/whatsnew/v0.24.0.txt

@@ -683,6 +683,7 @@ Plotting
 
 - Bug in :func:`DataFrame.plot.scatter` and :func:`DataFrame.plot.hexbin` caused x-axis label and ticklabels to disappear when colorbar was on in IPython inline backend (:issue:`10611`, :issue:`10678`, and :issue:`20455`)
 - Bug in plotting a Series with datetimes using :func:`matplotlib.axes.Axes.scatter` (:issue:`22039`)
+- Bug in colors selection in :func:`DataFrame.plot.bar` which causes different colors to be used when no color is specified (:issue:`20585`)
 
 Groupby/Resample/Rolling
 ^^^^^^^^^^^^^^^^^^^^^^^^

pandas/plotting/_core.py

@@ -1233,6 +1233,16 @@ def _plot(cls, ax, x, y, w, start=0, log=False, **kwds):
     def _start_base(self):
         return self.bottom
 
+    def _get_colors(self, num_colors=None, color_kwds='color'):
+        color = self.kwds.get(color_kwds)
+        if color is not None:
+            return color
+        else:
+            num_colors = self.nseries
+            return _get_standard_colors(num_colors=num_colors,
+                                        colormap=self.colormap,
+                                        color=self.kwds.get(color_kwds))
+
     def _make_plot(self):
         import matplotlib as mpl
 

pandas/plotting/_style.py

@@ -91,7 +91,9 @@ def _maybe_valid_colors(colors):
             # mpl will raise error any of them is invalid
             pass
 
-    if len(colors) != num_colors:
+    if len(colors) > num_colors:
+        colors = colors[:num_colors]
+    elif len(colors) < num_colors:
         try:
             multiple = num_colors // len(colors) - 1
         except ZeroDivisionError:

pandas/tests/plotting/test_series.py

@@ -21,6 +21,7 @@
 from pandas.tests.plotting.common import (TestPlotBase, _check_plot_works,
                                           _skip_if_no_scipy_gaussian_kde,
                                           _ok_for_gaussian_kde)
+from pandas.plotting._style import _get_standard_colors
 
 
 @td.skip_if_no_mpl
@@ -268,6 +269,36 @@ def test_bar_user_colors(self):
                     (1., 0., 0., 1.)]
         assert result == expected
 
+    def test_bar_user_colors_limited(self):
+        # GH 20585
+        s = Series([1, 2, 3, 4])
+        ax = s.plot.bar(color=['red', 'blue'])
+        result = [p.get_facecolor() for p in ax.patches]
+        expected = [(1., 0., 0., 1.),
+                    (0., 0., 1., 1.),
+                    (1., 0., 0., 1.),
+                    (0., 0., 1., 1.)]
+        assert result == expected
+
+    def test_bar_user_colors_more_limited(self):
+        s = Series([1, 2, 3, 4, 5, 6])
+        ax = s.plot.bar(color=['red', 'blue'])
+        result = [p.get_facecolor() for p in ax.patches]
+        expected = [(1., 0., 0., 1.),
+                    (0., 0., 1., 1.),
+                    (1., 0., 0., 1.),
+                    (0., 0., 1., 1.),
+                    (1., 0., 0., 1.),
+                    (0., 0., 1., 1.)]
+        assert result == expected
+
+    @pytest.mark.parametrize("num_colors",range(0,15))
+    def test_standard_color(self, num_colors):
+        # GH 20585
+        colors = _get_standard_colors(num_colors=num_colors)
+
+        assert len(colors) == num_colors
+
     def test_rotation(self):
         df = DataFrame(randn(5, 5))
         # Default rot 0