ENH: Make the ability to resample interpolated colormaps public

Author: tacaswell

doc/users/next_whats_new/resample_colormaps.rst

@@ -0,0 +1,6 @@
+Make the resample method on `.Colormap` instances public
+--------------------------------------------------------
+
+On `.LinearSegmentedColormap` and `.ListedColormap` the previously private
+``_resample`` method is made public as `.Colormap.resampled`.  This method
+creates a new `.Colormap` instance with the specified lookup table size.

lib/matplotlib/cm.py

@@ -269,7 +269,7 @@ def get_cmap(name=None, lut=None):
     if lut is None:
         return _colormaps[name]
     else:
-        return _colormaps[name]._resample(lut)
+        return _colormaps[name].resample(lut)
 
 
 def unregister_cmap(name):

lib/matplotlib/colors.py

@@ -854,8 +854,17 @@ def is_gray(self):
         return (np.all(self._lut[:, 0] == self._lut[:, 1]) and
                 np.all(self._lut[:, 0] == self._lut[:, 2]))
 
-    def _resample(self, lutsize):
+    def resampled(self, lutsize):
         """Return a new colormap with *lutsize* entries."""
+        if hasattr(self, '_resample'):
+            _api.warn_external(
+                "The ability to resample a color map is now public API "
+                f"However the class {type(self)} still only implements "
+                "the previous private _resample method.  Please update "
+                "your class."
+            )
+            return self._resample(lutsize)
+
         raise NotImplementedError()
 
     def reversed(self, name=None):
@@ -1050,7 +1059,7 @@ def from_list(name, colors, N=256, gamma=1.0):
 
         return LinearSegmentedColormap(name, cdict, N, gamma)
 
-    def _resample(self, lutsize):
+    def resampled(self, lutsize):
         """Return a new colormap with *lutsize* entries."""
         new_cmap = LinearSegmentedColormap(self.name, self._segmentdata,
                                            lutsize)
@@ -1154,7 +1163,7 @@ def _init(self):
         self._isinit = True
         self._set_extremes()
 
-    def _resample(self, lutsize):
+    def resampled(self, lutsize):
         """Return a new colormap with *lutsize* entries."""
         colors = self(np.linspace(0, 1, lutsize))
         new_cmap = ListedColormap(colors, name=self.name)

lib/matplotlib/tests/test_colors.py

@@ -31,7 +31,7 @@ def test_create_lookup_table(N, result):
 
 def test_resample():
     """
-    GitHub issue #6025 pointed to incorrect ListedColormap._resample;
+    GitHub issue #6025 pointed to incorrect ListedColormap.resample;
     here we test the method for LinearSegmentedColormap as well.
     """
     n = 101
@@ -47,8 +47,8 @@ def test_resample():
         cmap.set_under('r')
         cmap.set_over('g')
         cmap.set_bad('b')
-    lsc3 = lsc._resample(3)
-    lc3 = lc._resample(3)
+    lsc3 = lsc.resample(3)
+    lc3 = lc.resample(3)
     expected = np.array([[0.0, 0.2, 1.0, 0.7],
                          [0.5, 0.2, 0.5, 0.7],
                          [1.0, 0.2, 0.0, 0.7]], float)