Allow constructing empty HoloViews pane (#307)

panel/pane/holoviews.py

@@ -10,6 +10,7 @@
 from functools import partial
 
 import param
+from bokeh.models import Spacer as _BkSpacer
 
 from ..io import state
 from ..layout import Panel, Column
@@ -45,7 +46,7 @@ class HoloViews(PaneBase):
 
     _panes = {'bokeh': Bokeh, 'matplotlib': Matplotlib, 'plotly': Plotly}
 
-    def __init__(self, object, **params):
+    def __init__(self, object=None, **params):
         super(HoloViews, self).__init__(object, **params)
         self.widget_box = Column()
         self._update_widgets()
@@ -114,14 +115,17 @@ def _get_model(self, doc, root=None, parent=None, comm=None):
         if root is None:
             return self._get_root(doc, comm)
         ref = root.ref['id']
-        plot = self._render(doc, comm, root)
-        child_pane = self._panes.get(self.backend, Pane)(plot.state)
-        model = child_pane._get_model(doc, root, parent, comm)
-        if ref in self._plots:
-            old_plot, old_pane = self._plots[ref]
-            old_plot.comm = None # Ensure comm does not cleaned up
-            old_plot.cleanup()
-        self._plots[ref] = (plot, child_pane)
+        if self.object is None:
+            model = _BkSpacer()
+        else:
+            plot = self._render(doc, comm, root)
+            child_pane = self._panes.get(self.backend, Pane)(plot.state)
+            model = child_pane._get_model(doc, root, parent, comm)
+            if ref in self._plots:
+                old_plot, old_pane = self._plots[ref]
+                old_plot.comm = None # Ensure comm does not cleaned up
+                old_plot.cleanup()
+            self._plots[ref] = (plot, child_pane)
         self._models[ref] = (model, parent)
         return model
 
@@ -245,9 +249,13 @@ def generate_panel_bokeh_map(root_model, panel_views):
     mapping panel elements to its bokeh models
     """
     map_hve_bk = defaultdict(list)
+    ref = root_model.ref['id']
     for pane in panel_views:
         if root_model.ref['id'] in pane._models:
-            bk_plots = pane._plots[root_model.ref['id']][0].traverse(lambda x: x, [is_bokeh_element_plot])
+            plot, subpane = pane._plots.get(ref, (None, None))
+            if plot is None:
+                continue
+            bk_plots = plot.traverse(lambda x: x, [is_bokeh_element_plot])
             for plot in bk_plots:
                 for hv_elem in plot.link_sources:
                     map_hve_bk[hv_elem].append(plot)

panel/tests/test_holoviews.py

@@ -7,7 +7,7 @@
 
 from bokeh.models import (Row as BkRow, Column as BkColumn, GlyphRenderer,
                           Scatter, Line, GridBox, Select as BkSelect,
-                          Slider as BkSlider)
+                          Slider as BkSlider, Spacer as BkSpacer)
 from bokeh.plotting import Figure
 
 from panel.layout import Column, Row
@@ -113,6 +113,24 @@ def test_holoviews_pane_bokeh_renderer(document, comm):
     assert pane._models == {}
 
 
+@pytest.mark.usefixtures("hv_bokeh")
+@hv_available
+def test_holoviews_pane_initialize_empty(document, comm):
+    pane = HoloViews()
+
+    # Create pane
+    row = pane._get_root(document, comm=comm)
+
+    assert isinstance(row, BkRow)
+    assert len(row.children) == 1
+    model = row.children[0]
+    assert isinstance(model, BkSpacer)
+
+    pane.object = hv.Curve([1, 2, 3])
+    model = row.children[0]
+    assert isinstance(model, Figure)
+
+
 @hv_available
 def test_holoviews_widgets_from_dynamicmap(document, comm):
     range_dim = hv.Dimension('A', range=(0, 10.))