Raise error if cmap is list of colors

doc/whats-new.rst

@@ -34,6 +34,8 @@ Breaking changes
 - :py:func:`~xarray.concat` now requires the ``dim`` argument. Its ``indexers``, ``mode``
   and ``concat_over`` kwargs have now been removed.
   By `Deepak Cherian <https://github.com/dcherian>`_
+- Passing a list of colors in ``cmap`` will now raise an error, having been deprecated since
+  v0.6.1.
 - Most xarray objects now define ``__slots__``. This reduces overall RAM usage by ~22%
   (not counting the underlying numpy buffers); on CPython 3.7/x64, a trivial DataArray
   has gone down from 1.9kB to 1.5kB.

xarray/plot/utils.py

@@ -737,11 +737,9 @@ def _process_cmap_cbar_kwargs(
     # we should not be getting a list of colors in cmap anymore
     # is there a better way to do this test?
     if isinstance(cmap, (list, tuple)):
-        warnings.warn(
+        raise ValueError(
             "Specifying a list of colors in cmap is deprecated. "
-            "Use colors keyword instead.",
-            DeprecationWarning,
-            stacklevel=3,
+            "Use colors keyword instead."
         )
 
     cmap_kwargs = {

xarray/tests/test_plot.py

@@ -1320,8 +1320,8 @@ def test_cmap_and_color_both(self):
         with pytest.raises(ValueError):
             self.plotmethod(colors="k", cmap="RdBu")
 
-    def list_of_colors_in_cmap_deprecated(self):
-        with pytest.raises(Exception):
+    def list_of_colors_in_cmap_raises_error(self):
+        with raises_regex(ValueError, "list of colors"):
             self.plotmethod(cmap=["k", "b"])
 
     @pytest.mark.slow