warning when scatter plot color settings discarded (matplotlib#23516)

* Warning when scatter plot color settings discarded

* Update lib/matplotlib/axes/_axes.py

Co-authored-by: Tim Hoffmann <[email protected]>

* Wrapped 23516-MS.rst lines at 80 characters

* Fixed tests to look for proper warning message

* Update doc/api/next_api_changes/behavior/23516-MS.rst

Co-authored-by: Elliott Sales de Andrade <[email protected]>

Co-authored-by: Tim Hoffmann <[email protected]>
Co-authored-by: Elliott Sales de Andrade <[email protected]>

Author: 3 people

doc/api/next_api_changes/behavior/23516-MS.rst

@@ -0,0 +1,6 @@
+Warning when scatter plot color settings discarded
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+When making an animation of a scatter plot, if you don't set *c* (the color
+value parameter) when initializing the artist, the color settings are ignored.
+`.Axes.scatter` now raises a warning if color-related settings are changed
+without setting *c*.

lib/matplotlib/axes/_axes.py

@@ -4578,6 +4578,16 @@ def scatter(self, x, y, s=None, c=None, marker=None, cmap=None, norm=None,
             collection.set_cmap(cmap)
             collection.set_norm(norm)
             collection._scale_norm(norm, vmin, vmax)
+        else:
+            extra_kwargs = {
+                    'cmap': cmap, 'norm': norm, 'vmin': vmin, 'vmax': vmax
+                    }
+            extra_keys = [k for k, v in extra_kwargs.items() if v is not None]
+            if any(extra_keys):
+                keys_str = ", ".join(f"'{k}'" for k in extra_keys)
+                _api.warn_external(
+                    "No data for colormapping provided via 'c'. "
+                    f"Parameters {keys_str} will be ignored")
         collection._internal_update(kwargs)
 
         # Classic mode only:

lib/matplotlib/tests/test_axes.py

@@ -2383,6 +2383,25 @@ def test_scatter_color(self):
         with pytest.raises(ValueError):
             plt.scatter([1, 2, 3], [1, 2, 3], color=[1, 2, 3])
 
+    @pytest.mark.parametrize('kwargs',
+                                [
+                                    {'cmap': 'gray'},
+                                    {'norm': mcolors.Normalize()},
+                                    {'vmin': 0},
+                                    {'vmax': 0}
+                                ])
+    def test_scatter_color_warning(self, kwargs):
+        warn_match = "No data for colormapping provided "
+        # Warn for cases where 'cmap', 'norm', 'vmin', 'vmax'
+        # kwargs are being overridden
+        with pytest.warns(Warning, match=warn_match):
+            plt.scatter([], [], **kwargs)
+        with pytest.warns(Warning, match=warn_match):
+            plt.scatter([1, 2], [3, 4], c=[], **kwargs)
+        # Do not warn for cases where 'c' matches 'x' and 'y'
+        plt.scatter([], [], c=[], **kwargs)
+        plt.scatter([1, 2], [3, 4], c=[4, 5], **kwargs)
+
     def test_scatter_unfilled(self):
         coll = plt.scatter([0, 1, 2], [1, 3, 2], c=['0.1', '0.3', '0.5'],
                            marker=mmarkers.MarkerStyle('o', fillstyle='none'),

lib/matplotlib/tests/test_colorbar.py

@@ -280,7 +280,7 @@ def test_remove_from_figure(use_gridspec):
     Test `remove` with the specified ``use_gridspec`` setting
     """
     fig, ax = plt.subplots()
-    sc = ax.scatter([1, 2], [3, 4], cmap="spring")
+    sc = ax.scatter([1, 2], [3, 4])
     sc.set_array(np.array([5, 6]))
     pre_position = ax.get_position()
     cb = fig.colorbar(sc, use_gridspec=use_gridspec)
@@ -296,7 +296,7 @@ def test_remove_from_figure_cl():
     Test `remove` with constrained_layout
     """
     fig, ax = plt.subplots(constrained_layout=True)
-    sc = ax.scatter([1, 2], [3, 4], cmap="spring")
+    sc = ax.scatter([1, 2], [3, 4])
     sc.set_array(np.array([5, 6]))
     fig.draw_without_rendering()
     pre_position = ax.get_position()