FIX: pass cmap name string not tuple to interactive topomap slider kwargs (#14039)

Author: PragnyaKhandelwal

mne/viz/tests/test_topomap.py

@@ -353,6 +353,25 @@ def test_plot_evoked_topomap_border():
     assert_almost_equal(img_data[idx, idx], data[0], decimal=9)
 
 
+def test_plot_topomap_interactive_slider_cmap():
+    """Regression test: moving the time slider must not raise KeyError on cmap.
+
+    gh-14038: group_cmaps stored full (name, draggable) tuples but the slider
+    callback passed the tuple as the cmap kwarg, causing a KeyError in
+    matplotlib's colormap registry.
+    """
+    evoked = read_evokeds(evoked_fname, "Left Auditory", baseline=(None, 0))
+    evoked.pick("mag")
+    fig = evoked.plot_topomap(times="interactive", **fast_test)
+    # Trigger the slider callback — this crashed before the fix.
+    # The slider axes is not necessarily fig.axes[-1] (colorbar may follow it),
+    # so find it by looking for the axes that has a .slider attribute.
+    slider_ax = next(ax for ax in fig.axes if hasattr(ax, "slider"))
+    slider_ax.slider.set_val(evoked.times[len(evoked.times) // 2])
+    # Verify the topomap was redrawn (not blank) — would fail with double-scaling
+    assert len(fig.axes[0].images) > 0
+
+
 @pytest.mark.slowtest
 def test_plot_topomap_basic():
     """Test basics of topomap plotting."""

mne/viz/topomap.py

@@ -2724,7 +2724,7 @@ def _plot_evoked_topomap(
     if interactive:
         # Add a slider to the figure and start publishing and subscribing to time_change
         # events.
-        kwargs.update(vlim=group_vlims[0], cmap=group_cmaps[0])
+        kwargs.update(vlim=group_vlims[0], cmap=group_cmaps[0][0])
         axes.append(fig.add_subplot(gs[1]))
         slider = Slider(
             axes[-1],