Merge pull request matplotlib#6211 from kjartankg/reverse-cm

ENH: A method added to Colormap classes to reverse the colormap

Closes matplotlib#4271

Author: tacaswell

doc/users/whats_new/reversed_colormap.rst

@@ -0,0 +1,7 @@
+Colormap reversed method
+------------------------
+
+The methods :meth:`~matplotlib.colors.LinearSegmentedColormap.reversed` and
+:meth:`~matplotlib.colors.ListedColormap.reversed` return a reversed
+instance of the Colormap. This implements a way for any Colormap to be
+reversed.

lib/matplotlib/colors.py

@@ -665,6 +665,25 @@ def _resample(self, lutsize):
         """
         raise NotImplementedError()
 
+    def reversed(self, name=None):
+        """
+        Make a reversed instance of the Colormap.
+
+        .. note :: Function not implemented for base class.
+
+        Parameters
+        ----------
+        name : str, optional
+            The name for the reversed colormap. If it's None the
+            name will be the name of the parent colormap + "_r".
+
+        Notes
+        -----
+        See :meth:`LinearSegmentedColormap.reversed` and
+        :meth:`ListedColormap.reversed`
+        """
+        raise NotImplementedError()
+
 
 class LinearSegmentedColormap(Colormap):
     """Colormap objects based on lookup tables using linear segments.
@@ -784,6 +803,40 @@ def _resample(self, lutsize):
         """
         return LinearSegmentedColormap(self.name, self._segmentdata, lutsize)
 
+    def reversed(self, name=None):
+        """
+        Make a reversed instance of the Colormap.
+
+        Parameters
+        ----------
+        name : str, optional
+            The name for the reversed colormap. If it's None the
+            name will be the name of the parent colormap + "_r".
+
+        Returns
+        -------
+        LinearSegmentedColormap
+            The reversed colormap.
+        """
+        if name is None:
+            name = self.name + "_r"
+
+        # Function factory needed to deal with 'late binding' issue.
+        def factory(dat):
+            def func_r(x):
+                return dat(1.0 - x)
+            return func_r
+
+        data_r = dict()
+        for key, data in six.iteritems(self._segmentdata):
+            if six.callable(data):
+                data_r[key] = factory(data)
+            else:
+                new_data = [(1.0 - x, y1, y0) for x, y0, y1 in reversed(data)]
+                data_r[key] = new_data
+
+        return LinearSegmentedColormap(name, data_r, self.N, self._gamma)
+
 
 class ListedColormap(Colormap):
     """Colormap object generated from a list of colors.
@@ -856,6 +909,27 @@ def _resample(self, lutsize):
         colors = self(np.linspace(0, 1, lutsize))
         return ListedColormap(colors, name=self.name)
 
+    def reversed(self, name=None):
+        """
+        Make a reversed instance of the Colormap.
+
+        Parameters
+        ----------
+        name : str, optional
+            The name for the reversed colormap. If it's None the
+            name will be the name of the parent colormap + "_r".
+
+        Returns
+        -------
+        ListedColormap
+            A reversed instance of the colormap.
+        """
+        if name is None:
+            name = self.name + "_r"
+
+        colors_r = list(reversed(self.colors))
+        return ListedColormap(colors_r, name=name, N=self.N)
+
 
 class Normalize(object):
     """

lib/matplotlib/tests/test_colors.py

@@ -588,6 +588,18 @@ def test_pandas_iterable():
     assert_sequence_equal(cm1.colors, cm2.colors)
 
 
+def test_colormap_reversing():
+    """Check the generated _lut data of a colormap and corresponding
+    reversed colormap if they are almost the same."""
+    for name in six.iterkeys(cm.cmap_d):
+        cmap = plt.get_cmap(name)
+        cmap_r = cmap.reversed()
+        if not cmap_r._isinit:
+            cmap._init()
+            cmap_r._init()
+        assert_array_almost_equal(cmap._lut[:-3], cmap_r._lut[-4::-1])
+
+
 if __name__ == '__main__':
     import nose
     nose.runmodule(argv=['-s', '--with-doctest'], exit=False)