From ebef85d2a0a15926c07ea02a8dea11a41c1cbcd2 Mon Sep 17 00:00:00 2001
From: Antony Lee <anntzer.lee@gmail.com>
Date: Wed, 17 Apr 2019 12:20:34 +0200
Subject: [PATCH] Backport PR #13966: Fix colorbar setting without artist

---
 lib/matplotlib/cm.py                  | 11 +++++++++++
 lib/matplotlib/tests/test_colorbar.py | 16 +++++++++++++---
 2 files changed, 24 insertions(+), 3 deletions(-)

diff --git a/lib/matplotlib/cm.py b/lib/matplotlib/cm.py
index 26b1766e84dd..94cf2ceee24c 100644
--- a/lib/matplotlib/cm.py
+++ b/lib/matplotlib/cm.py
@@ -92,6 +92,7 @@ def _generate_cmap(name, lutsize):
     else:
         return colors.LinearSegmentedColormap.from_list(name, spec, lutsize)
 
+
 LUTSIZE = mpl.rcParams['image.lut']
 
 # Generate the reversed specifications (all at once, to avoid
@@ -331,6 +332,16 @@ def set_clim(self, vmin=None, vmax=None):
             self.norm.vmax = colors._sanitize_extrema(vmax)
         self.changed()
 
+    def get_alpha(self):
+        """
+        Returns
+        -------
+        alpha : float
+            Always returns 1.
+        """
+        # This method is intended to be overridden by Artist sub-classes
+        return 1.
+
     def set_cmap(self, cmap):
         """
         set the colormap for luminance data
diff --git a/lib/matplotlib/tests/test_colorbar.py b/lib/matplotlib/tests/test_colorbar.py
index 0244101ebcb0..c65ad02c31a5 100644
--- a/lib/matplotlib/tests/test_colorbar.py
+++ b/lib/matplotlib/tests/test_colorbar.py
@@ -1,12 +1,14 @@
 import numpy as np
 import pytest
 
+from matplotlib import cm
+import matplotlib.colors as mcolors
+
 from matplotlib import rc_context
 from matplotlib.testing.decorators import image_comparison
 import matplotlib.pyplot as plt
 from matplotlib.colors import (BoundaryNorm, LogNorm, PowerNorm, Normalize,
                                DivergingNorm)
-from matplotlib.cm import get_cmap
 from matplotlib.colorbar import ColorbarBase, _ColorbarLogLocator
 from matplotlib.ticker import LogLocator, LogFormatter, FixedLocator
 
@@ -20,7 +22,7 @@ def _get_cmap_norms():
     colorbar_extension_length.
     """
     # Create a color map and specify the levels it represents.
-    cmap = get_cmap("RdBu", lut=5)
+    cmap = cm.get_cmap("RdBu", lut=5)
     clevs = [-5., -2.5, -.5, .5, 1.5, 3.5]
     # Define norms for the color maps.
     norms = dict()
@@ -240,7 +242,7 @@ def test_colorbar_closed_patch():
     ax4 = fig.add_axes([0.05, 0.25, 0.9, 0.1])
     ax5 = fig.add_axes([0.05, 0.05, 0.9, 0.1])
 
-    cmap = get_cmap("RdBu", lut=5)
+    cmap = cm.get_cmap("RdBu", lut=5)
 
     im = ax1.pcolormesh(np.linspace(0, 10, 16).reshape((4, 4)), cmap=cmap)
 
@@ -558,3 +560,11 @@ def test_extend_colorbar_customnorm():
     cb = fig.colorbar(pcm, ax=ax[0], extend='both')
     np.testing.assert_allclose(cb.ax.get_position().extents,
                                [0.78375, 0.536364, 0.796147, 0.9], rtol=1e-3)
+
+
+def test_mappable_no_alpha():
+    fig, ax = plt.subplots()
+    sm = cm.ScalarMappable(norm=mcolors.Normalize(), cmap='viridis')
+    fig.colorbar(sm)
+    sm.set_cmap('plasma')
+    plt.draw()