Address adversarial review findings for per-channel norm

1. Improve error message when norm list + cmap resolution fails —
   now mentions cmap/channel mismatch as possible cause instead of
   the misleading "multiple colormaps are used"

2. Unwrap length-1 cmap_params lists to scalar so single-channel
   images still take the fast path (proper norm in imshow + colorbar)

3. Make Normalize copy unconditional in the multi-channel compositing
   loop — eliminates fragile conditional that only copied auto-ranging
   norms, preventing any future cross-channel mutation bugs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/basic.py

@@ -658,15 +658,20 @@ def render_images(
             else:
                 if isinstance(norm, list):
                     raise ValueError(
-                        "Parameter 'norm' can only be a list when multiple colormaps are used. "
-                        "Pass a list of colormaps via 'cmap' or use a single Normalize."
+                        "Parameter 'norm' can only be a list when a matching list of colormaps is resolved. "
+                        "Pass an explicit list of colormaps via 'cmap' whose length matches the number of "
+                        "channels, or use a single Normalize."
                     )
                 cmap_params = _prepare_cmap_norm(
                     cmap=cmap,
                     norm=norm,
                     na_color=param_values["na_color"],
                     **kwargs,
                 )
+            # Unwrap length-1 list so the single-channel rendering path
+            # (which checks `not isinstance(cmap_params, list)`) still works.
+            if isinstance(cmap_params, list) and len(cmap_params) == 1:
+                cmap_params = cmap_params[0]
             sdata.plotting_tree[f"{n_steps + 1}_render_images"] = ImageRenderParams(
                 element=element,
                 channel=param_values["channel"],

src/spatialdata_plot/pl/render.py

@@ -1313,8 +1313,8 @@ def _render_images(
             else:
                 ch_norm = render_params.cmap_params.norm
 
-            # Auto-ranging norms are stateful — copy so each channel normalizes independently
-            if isinstance(ch_norm, Normalize) and (ch_norm.vmin is None or ch_norm.vmax is None):
+            # Normalize objects are stateful — always copy to prevent cross-channel mutation
+            if isinstance(ch_norm, Normalize):
                 ch_norm = copy(ch_norm)
 
             layers[ch] = ch_norm(layers[ch])

tests/pl/test_render_images.py

@@ -478,5 +478,5 @@ def test_norm_list_with_invalid_element_raises():
 def test_norm_list_without_cmap_list_raises():
     """Norm list requires explicit cmap list."""
     sdata = _make_multichannel_sdata()
-    with pytest.raises(ValueError, match="can only be a list when multiple colormaps"):
+    with pytest.raises(ValueError, match="can only be a list when a matching list of colormaps"):
         sdata.pl.render_images("img", norm=[Normalize(0, 1)] * 3).pl.show()