fix: setting selection_expr programmatically

holoviews/selection.py

@@ -289,6 +289,8 @@ def instance(self_or_cls, **params):
         inst._obj_selections = {}
         inst._obj_regions = {}
         inst._reset_regions = True
+        inst._user_show_regions = inst.show_regions
+        inst._updating_show_regions_internal = False
 
         # _datasets caches
         inst._datasets = []
@@ -298,6 +300,12 @@ def instance(self_or_cls, **params):
 
         return inst
 
+    @param.depends('show_regions', watch=True)
+    def _update_user_show_regions(self):
+        if self._updating_show_regions_internal:
+            return
+        self._user_show_regions = self.show_regions
+
     @param.depends('selection_expr', watch=True)
     def _update_pipes(self):
         sel_expr = self.selection_expr
@@ -392,10 +400,17 @@ def update_selection_expr(*_):
             new_selection_expr = inst.selection_expr
             current_selection_expr = inst._cross_filter_stream.selection_expr
             if repr(new_selection_expr) != repr(current_selection_expr):
+                # Reset the streams
+                for s in inst._selection_expr_streams.values():
+                    s.reset()
+                    s.event()
                 # Disable regions if setting selection_expr directly
-                if inst.show_regions:
+                if inst._user_show_regions:
+                    inst._updating_show_regions_internal = True
                     inst.show_regions = False
+                    inst._updating_show_regions_internal = False
                 inst._selection_override.event(selection_expr=new_selection_expr)
+                inst._cross_filter_stream.selection_expr = new_selection_expr
 
         inst.param.watch(
             update_selection_expr, ['selection_expr']
@@ -405,6 +420,10 @@ def selection_expr_changed(*_):
             new_selection_expr = inst._cross_filter_stream.selection_expr
             if repr(inst.selection_expr) != repr(new_selection_expr):
                 inst.selection_expr = new_selection_expr
+                if inst._user_show_regions:
+                    inst._updating_show_regions_internal = True
+                    inst.show_regions = True
+                    inst._updating_show_regions_internal = False
 
         inst._cross_filter_stream.param.watch(
             selection_expr_changed, ['selection_expr']
@@ -415,7 +434,6 @@ def selection_expr_changed(*_):
             def clear_stream_history(resetting, stream=stream):
                 if resetting:
                     stream.clear_history()
-            print("registering reset for ", stream)
             stream.plot_reset_stream.param.watch(
                 clear_stream_history, ['resetting']
             )

holoviews/tests/test_selection.py

@@ -175,6 +175,20 @@ def test_layout_selection_points_table(self):
             ]
         )
 
+    def test_select_expr_show_regions(self):
+        lnk_sel = link_selections.instance()
+        self.assertTrue(lnk_sel.show_regions)
+        se = (
+            (hv.dim('x') >= 0) & (hv.dim('x') <= 1) &
+            (hv.dim('y') >= 0) & (hv.dim('y') <= 1)
+        )
+        lnk_sel.selection_expr = se
+        self.assertFalse(lnk_sel.show_regions)
+        lnk_sel.selection_expr = None
+        self.assertFalse(lnk_sel.show_regions)
+        lnk_sel._cross_filter_stream.selection_expr = se
+        self.assertTrue(lnk_sel.show_regions)
+
     def test_overlay_points_errorbars(self, dynamic=False):
         points = Points(self.data)
         error = ErrorBars(self.data, kdims='x', vdims=['y', 'e'])

holoviews/tests/ui/bokeh/test_link_selections.py

@@ -0,0 +1,64 @@
+
+import pytest
+
+import holoviews as hv
+from holoviews.plotting.bokeh.renderer import BokehRenderer
+from holoviews.selection import link_selections
+
+from .. import expect
+
+pytestmark = pytest.mark.ui
+
+
+@pytest.mark.usefixtures("bokeh_backend")
+def test_link_selections_programmatic_clear_removes_region(serve_hv):
+    points = hv.Points([1, 2, 3, 4, 5, 6, 7, 8]).opts(width=400, height=300)
+
+    # Helper to inspect bokeh model renderers for region-like glyphs
+    def count_highlighted_region_renderers(bokeh_plot):
+        cnt = 0
+        for r in bokeh_plot.renderers:
+            data = r.data_source.data
+            if len(data.get("left", [])) > 0:
+                cnt += 1
+                continue
+        return cnt
+
+    ls = link_selections.instance()
+    linked = ls(points).opts(active_tools=["box_select"])
+
+    page = serve_hv(linked)
+    hv_plot = page.locator(".bk-events")
+    expect(hv_plot).to_have_count(1)
+    bbox = hv_plot.bounding_box()
+
+    page.wait_for_timeout(300)
+    initial_count = count_highlighted_region_renderers(BokehRenderer.get_plot(linked[()]).state)
+    assert initial_count == 0
+
+    # Box-drag selection
+    start_x = bbox["x"] + bbox["width"] * 0.25
+    start_y = bbox["y"] + bbox["height"] * 0.25
+    end_x = bbox["x"] + bbox["width"] * 0.75
+    end_y = bbox["y"] + bbox["height"] * 0.75
+
+    hv_plot.click()
+    page.mouse.move(start_x, start_y)
+    page.mouse.down(button="left")
+    page.mouse.move(end_x, end_y, steps=10)
+    page.mouse.up(button="left")
+
+    page.wait_for_timeout(200)
+
+    assert ls.selection_expr is not None
+
+    # Ensure at least one new region renderer was created by the interaction
+    post_select_count = count_highlighted_region_renderers(BokehRenderer.get_plot(linked[()]).state)
+    assert post_select_count == initial_count + 1
+
+    ls.selection_expr = None
+    page.wait_for_timeout(200)
+
+    # Final region count should be back to initial
+    final_count = count_highlighted_region_renderers(BokehRenderer.get_plot(linked[()]).state)
+    assert final_count == initial_count