MNT: raise ValueError and TypeError rather than KeyError

doc/api/prev_api_changes/api_changes_3.6.0/deprecations.rst

@@ -57,8 +57,7 @@ functions have been marked for pending deprecation:
 
   **Added 3.6.1** Use `matplotlib.cm.ColormapRegistry.get_cmap` if you
   have a string, `None` or a `matplotlib.colors.Colormap` object that you want
-  to convert to a `matplotlib.colors.Colormap` instance.  Raises `KeyError`
-  rather than `ValueError` for missing strings.
+  to convert to a `matplotlib.colors.Colormap` instance.
 - ``matplotlib.cm.register_cmap``; use `matplotlib.colormaps.register
   <.ColormapRegistry.register>` instead
 - ``matplotlib.cm.unregister_cmap``; use `matplotlib.colormaps.unregister

lib/matplotlib/cm.py

@@ -202,10 +202,6 @@ def get_cmap(self, cmap):
         Returns
         -------
         Colormap
-
-        Raises
-        ------
-        KeyError
         """
         # get the default color map
         if cmap is None:
@@ -214,9 +210,14 @@ def get_cmap(self, cmap):
         # if the user passed in a Colormap, simply return it
         if isinstance(cmap, colors.Colormap):
             return cmap
-
-        # otherwise, it must be a string so look it up
-        return self[cmap]
+        if isinstance(cmap, str):
+            _api.check_in_list(sorted(_colormaps), cmap=cmap)
+            # otherwise, it must be a string so look it up
+            return self[cmap]
+        raise TypeError(
+            'get_cmap expects None or an instance of a str or Colormap . ' +
+            f'you passed {cmap!r} of type {type(cmap)}'
+        )
 
 
 # public access to the colormaps should be via `matplotlib.colormaps`. For now,

lib/matplotlib/tests/test_colors.py

@@ -111,18 +111,23 @@ def test_register_cmap():
 
 def test_colormaps_get_cmap():
     cr = mpl.colormaps
-    new_cm = mcolors.ListedColormap(cr["viridis"].colors, name='v2')
 
-    # check None, str, and Colormap pass
+    # check str, and Colormap pass
     assert cr.get_cmap('plasma') == cr["plasma"]
     assert cr.get_cmap(cr["magma"]) == cr["magma"]
 
-    # check default default
+    # check default
     assert cr.get_cmap(None) == cr[mpl.rcParams['image.cmap']]
+
+    # check ValueError on bad name
     bad_cmap = 'AardvarksAreAwkward'
-    with pytest.raises(KeyError, match=bad_cmap):
+    with pytest.raises(ValueError, match=bad_cmap):
         cr.get_cmap(bad_cmap)
 
+    # check TypeError on bad type
+    with pytest.raises(TypeError, match='object'):
+        cr.get_cmap(object())
+
 
 def test_double_register_builtin_cmap():
     name = "viridis"