Allow and handle non-parameters in rasterize.instance() (#5811)

Co-authored-by: Simon Høxbro Hansen <simon.hansen@me.com>

holoviews/operation/datashader.py

@@ -1375,20 +1375,25 @@ class rasterize(AggregationOperation):
     ]
 
     __instance_params = set()
+    __instance_kwargs = {}
 
     @bothmethod
     def instance(self_or_cls, **params):
-        inst = super().instance(**params)
-        inst.__instance_params = set(params)
+        kwargs = set(params) - set(self_or_cls.param)
+        inst_params = {k: v for k, v in params.items() if k in self_or_cls.param}
+        inst = super().instance(**inst_params)
+        inst.__instance_params = set(inst_params)
+        inst.__instance_kwargs = {k: v for k, v in params.items() if k in kwargs}
         return inst
 
     def _process(self, element, key=None):
         # Potentially needs traverse to find element types first?
         all_allowed_kws = set()
         all_supplied_kws = set()
-        instance_params = {
-            k: getattr(self, k) for k in self.__instance_params
-        }
+        instance_params = dict(
+            self.__instance_kwargs,
+            **{k: getattr(self, k) for k in self.__instance_params}
+        )
         for predicate, transform in self._transforms:
             merged_param_values = dict(instance_params, **self.p)
 

holoviews/tests/operation/test_datashader.py

@@ -9,11 +9,13 @@
 from holoviews import (
     Dimension, Curve, Points, Image, Dataset, RGB, Path, Graph, TriMesh,
     QuadMesh, NdOverlay, Contours, Spikes, Spread, Area, Rectangles,
-    Segments, Polygons, Nodes
+    Segments, Polygons, Nodes, DynamicMap, Overlay
 )
+from holoviews.util import render
 from holoviews.streams import Tap
 from holoviews.element.comparison import ComparisonTestCase
 from numpy import nan
+from holoviews.operation import apply_when
 from packaging.version import Version
 
 try:
@@ -1169,6 +1171,26 @@ def test_rasterize_image_expand_default(self):
         output = img.data["z"].to_numpy()
         assert np.isnan(output).any()
 
+    def test_rasterize_apply_when_instance_with_line_width(self):
+        df = pd.DataFrame(
+            np.random.multivariate_normal(
+            (0, 0), [[0.1, 0.1], [0.1, 1.0]], (100,))
+        )
+        df.columns = ["a", "b"]
+
+        curve = Curve(df, kdims=["a"], vdims=["b"])
+        # line_width is not a parameter
+        custom_rasterize = rasterize.instance(line_width=2)
+        assert {'line_width': 2} == custom_rasterize._rasterize__instance_kwargs
+        output = apply_when(
+            curve, operation=custom_rasterize, predicate=lambda x: len(x) > 10
+        )
+        render(output, "bokeh")
+        assert isinstance(output, DynamicMap)
+        overlay = output.items()[0][1]
+        assert isinstance(overlay, Overlay)
+        assert len(overlay) == 2
+
 class DatashaderSpreadTests(ComparisonTestCase):
 
     def test_spread_rgb_1px(self):