fix: widget.link() to support HoloViews streams

[SuMayaBee]

Problem

Panel's widget.link() method is designed to create bidirectional connections between widgets and other parameterized objects. When we try to link a widget to a HoloViews stream (used for interactive plots), the connection appears to be made, but it doesn't actually work.

For example, when we write:

tabulator.link(selection_1d, selection="index", bidirectional=True)

We expect that clicking a row in the table will update the plot selection, and clicking a point in the plot will update the table selection. However, while the table selection changes correctly, the plot never updates because the stream's subscribers aren't notified.

The video demonstrates:

Before fix: Using a single tabulator.link() call doesn't work

PR3_Before.mp4

After fix: Using a single tabulator.link() call that works correctly

PR3_After.mp4

What's happening: The link() method uses setattr() to update the stream parameter. This sets the value correctly, but HoloViews streams need their .event() method to be called to notify all subscribers (like DynamicMap objects) that something changed. Without calling .event(), the stream value changes silently but nothing listening to the stream gets notified, so plots don't refresh.

The workaround required two separate pn.bind() calls:

def selection1d_to_tabulator(index):
    tabulator.selection = index

def tabulator_to_selection1d(selection):
    selection_1d.event(index=selection)

pn.bind(selection1d_to_tabulator, selection_1d.param.index, watch=True)
pn.bind(tabulator_to_selection1d, tabulator.param.selection, watch=True)

Solution

Modified Reactive.link() in panel/reactive.py to detect when the link target is a HoloViews Stream object. When detected, it calls .event() instead of setattr(), which properly triggers all stream subscribers and enables DynamicMap updates.

The fix is fully backward compatible — it only changes behavior for HoloViews stream targets. All other .link() calls work exactly as before.

Example

Before — required two pn.bind() calls:

pn.bind(selection1d_to_tabulator, selection_1d.param.index, watch=True)
pn.bind(tabulator_to_selection1d, tabulator.param.selection, watch=True)

After — one simple .link() call:

tabulator.link(selection_1d, selection="index", bidirectional=True)

Files Changed

• panel/reactive.py — Updated Reactive.link() to call .event() on HoloViews stream targets

Related Issue

Closes #7801

AI Disclosure: I used Gemini 3 Flash to explore and understand the codebase while working on this fix.

[codecov]

Codecov Report

❌ Patch coverage is 87.50000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.03%. Comparing base (49872f3) to head (2285dd7).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
panel/tests/test_reactive.py	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8450      +/-   ##
==========================================
- Coverage   86.12%   86.03%   -0.09%     
==========================================
  Files         349      349              
  Lines       54971    54986      +15     
==========================================
- Hits        47343    47308      -35     
- Misses       7628     7678      +50     

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[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.