API: remove implicit colormap generation in register_cmap

tacaswell · tacaswell · commit d28cbf2b2652 · 2020-09-24T21:13:45.000-04:00
This was deprecated in 3.3 via #15875 Enforce type of cmap as Colormap because we may not know that the object registered is of the wrong type until well after it was registered, defensively check here. We could do a duck-type check, but this is much simpler and we can cross the bridge of writing a duck-type checking function when someone has a use case for registering compatible-but-not-derived-from-Colormap instances.
diff --git a/lib/matplotlib/cm.py b/lib/matplotlib/cm.py
@@ -100,25 +100,27 @@ def _warn_deprecated(self):
 # Continue with definitions ...
 
 
-def register_cmap(name=None, cmap=None, data=None, lut=None):
+def register_cmap(name=None, cmap=None):
     """
     Add a colormap to the set recognized by :func:`get_cmap`.
 
-    It can be used in two ways::
+    Register a new colormap to be accessed by name ::
 
-        register_cmap(name='swirly', cmap=swirly_cmap)
+        LinearSegmentedColormap('swirly', data, lut)
+        register_cmap(cmap=swirly_cmap)
 
-        register_cmap(name='choppy', data=choppydata, lut=128)
+    Parameters
+    ----------
+    name : str, optional
+       The name that can be used in :func:`get_cmap` or :rc:`image.cmap`
 
-    In the first case, *cmap* must be a :class:`matplotlib.colors.Colormap`
-    instance.  The *name* is optional; if absent, the name will
-    be the :attr:`~matplotlib.colors.Colormap.name` attribute of the *cmap*.
+       If absent, the name will be the :attr:`~matplotlib.colors.Colormap.name`
+       attribute of the *cmap*.
+
+    cmap : matplotlib.colors.Colormap
+       Despite being the second argument and having a default value, this
+       is a required argument.
 
-    The second case is deprecated. Here, the three arguments are passed to
-    the :class:`~matplotlib.colors.LinearSegmentedColormap` initializer,
-    and the resulting colormap is registered. Instead of this implicit
-    colormap creation, create a `.LinearSegmentedColormap` and use the first
-    case: ``register_cmap(cmap=LinearSegmentedColormap(name, data, lut))``.
 
     Notes
     -----
@@ -134,23 +136,13 @@ def register_cmap(name=None, cmap=None, data=None, lut=None):
         except AttributeError as err:
             raise ValueError("Arguments must include a name or a "
                              "Colormap") from err
-    if isinstance(cmap, colors.Colormap):
-        cmap._global = True
-        _cmap_registry[name] = cmap
-        return
-    if lut is not None or data is not None:
-        cbook.warn_deprecated(
-            "3.3",
-            message="Passing raw data via parameters data and lut to "
-                    "register_cmap() is deprecated since %(since)s and will "
-                    "become an error %(removal)s. Instead use: register_cmap("
-                    "cmap=LinearSegmentedColormap(name, data, lut))")
-    # For the remainder, let exceptions propagate.
-    if lut is None:
-        lut = mpl.rcParams['image.lut']
-    cmap = colors.LinearSegmentedColormap(name, data, lut)
+
+    if not isinstance(cmap, colors.Colormap):
+        raise ValueError("You must pass a Colormap instance. "
+                         f"You passed {cmap} a {type(cmap)} object.")
     cmap._global = True
     _cmap_registry[name] = cmap
+    return
 
 
 def get_cmap(name=None, lut=None):
diff --git a/lib/matplotlib/tests/test_colors.py b/lib/matplotlib/tests/test_colors.py
@@ -72,6 +72,9 @@ def test_register_cmap():
                        match='Arguments must include a name or a Colormap'):
         cm.register_cmap()
 
+    with pytest.raises(ValueError, match="You must pass a Colormap instance."):
+        cm.register_cmap('nome', cmap='not a cmap')
+
 
 def test_colormap_global_set_warn():
     new_cm = plt.get_cmap('viridis')
