Make composite mapper dataset optional (#4879)

* make composite mapper dataset optional

* add test

* Explicit is better than implicit. (The Zen of Python)

---------

Co-authored-by: Tetsuo Koyama <tkoyama010@gmail.com>

pyvista/plotting/composite_mapper.py

@@ -521,7 +521,7 @@ class CompositePolyDataMapper(_vtk.vtkCompositePolyDataMapper2, _BaseMapper):
 
     Parameters
     ----------
-    dataset : pyvista.MultiBlock
+    dataset : pyvista.MultiBlock, optional
         Multiblock dataset.
 
     theme : pyvista.plotting.themes.Theme, optional
@@ -541,16 +541,14 @@ class CompositePolyDataMapper(_vtk.vtkCompositePolyDataMapper2, _BaseMapper):
     """
 
     def __init__(
-        self, dataset, theme=None, color_missing_with_nan=None, interpolate_before_map=None
+        self, dataset=None, theme=None, color_missing_with_nan=None, interpolate_before_map=None
     ):
         """Initialize this composite mapper."""
         super().__init__(theme=theme)
-        self.SetInputDataObject(dataset)
-
         # this must be added to set the color, opacity, and visibility of
         # individual blocks
         self._attr = CompositeAttributes(self, dataset)
-        self._dataset = dataset
+        self.dataset = dataset
 
         if color_missing_with_nan is not None:
             self.color_missing_with_nan = color_missing_with_nan
@@ -579,6 +577,12 @@ def dataset(self) -> 'pv.MultiBlock':  # numpydoc ignore=RT01
         """
         return self._dataset
 
+    @dataset.setter
+    def dataset(self, obj: 'pv.MultiBlock'):  # numpydoc ignore=GL08
+        self.SetInputDataObject(obj)
+        self._dataset = obj
+        self._attr._dataset = obj
+
     @property
     def block_attr(self) -> CompositeAttributes:  # numpydoc ignore=RT01
         """Return the block attributes.

tests/plotting/test_actor.py

@@ -17,6 +17,11 @@ def actor():
     return pv.Plotter().add_mesh(pv.Plane())
 
 
+@pytest.fixture()
+def actor_from_multi_block():
+    return pv.Plotter().add_mesh(pv.MultiBlock([pv.Plane()]))
+
+
 @pytest.fixture()
 def vol_actor():
     vol = pv.ImageData(dimensions=(10, 10, 10))
@@ -68,6 +73,14 @@ def test_actor_copy_shallow(actor):
     assert actor_copy.mapper is actor.mapper
 
 
+def test_actor_mblock_copy_shallow(actor_from_multi_block):
+    actor_copy = actor_from_multi_block.copy(deep=False)
+    assert actor_copy is not actor_from_multi_block
+    assert actor_copy.prop is actor_from_multi_block.prop
+    assert actor_copy.mapper is actor_from_multi_block.mapper
+    assert actor_copy.mapper.dataset is actor_from_multi_block.mapper.dataset
+
+
 @skip_mac
 def test_actor_texture(actor):
     texture = examples.download_masonry_texture()