Panel tries to set a numeric param to an HTML string when data is updated in the background with a Panel app in a notebook

[dennisjlee]

I ran into an unusual situation where I set up a simple Panel UI to run inside a Jupyter notebook, and then tried to update the data for the UI from a background thread. I observed that several updates would work properly, but at some point, Panel would try to update the value of a numeric param to be an HTML string (seemingly from a pn.widgets.StaticText) which would throw an exception and stop the UI from updating.

ALL software version info

Python: 3.11.9
OS: Mac OSX 14.5 on M3 Max
Browser: Chrome 126.0.6478.183 (Official Build) (arm64)

Selected python library versions:

bokeh==3.4.3
ipython==8.26.0
notebook==7.2.1
panel==1.4.4
param==2.1.1
pyviz_comms==3.0.2

Description of expected behavior and the observed behavior

It should be possible to update parameterized models from a background thread and have the UI update without issue.

Instead, when I trigger the background thread to update the model repeatedly, at some point Panel handles a message which is trying to set the model's value to be an HTML string, rather than the raw number.
It seems that this may come from the pn.widgets.StaticText widget - if I comment out and only display the position via pn.widgets.FloatSlider, I did not observe this problem in my repro case.

Complete, minimal, self-contained example code that reproduces the issue

See a minimal repro in repo here: https://github.com/dennisjlee/panel-thread-repro

Stack traceback and/or browser JavaScript console output

Traceback (most recent call last):
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/pyviz_comms/__init__.py", line 340, in _handle_msg
    self._on_msg(msg)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/panel/viewable.py", line 478, in _on_msg
    doc.unhold()
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/bokeh/document/document.py", line 776, in unhold
    self.callbacks.unhold()
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/bokeh/document/callbacks.py", line 432, in unhold
    self.trigger_on_change(event)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/bokeh/document/callbacks.py", line 409, in trigger_on_change
    invoke_with_curdoc(doc, event.callback_invoker)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/bokeh/document/callbacks.py", line 444, in invoke_with_curdoc
    return f()
           ^^^
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/bokeh/util/callback_manager.py", line 185, in invoke
    callback(attr, old, new)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/panel/reactive.py", line 474, in _comm_change
    state._handle_exception(e)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/panel/io/state.py", line 458, in _handle_exception
    raise exception
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/panel/reactive.py", line 472, in _comm_change
    self._schedule_change(doc, comm)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/panel/reactive.py", line 454, in _schedule_change
    self._change_event(doc)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/panel/reactive.py", line 450, in _change_event
    self._process_events(events)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/panel/reactive.py", line 387, in _process_events
    self.param.update(**self_params)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameterized.py", line 2319, in update
    restore = dict(self_._update(arg, **kwargs))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameterized.py", line 2352, in _update
    self_._batch_call_watchers()
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameterized.py", line 2546, in _batch_call_watchers
    self_._execute_watcher(watcher, events)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameterized.py", line 2506, in _execute_watcher
    watcher.fn(*args, **kwargs)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/panel/param.py", line 527, in link_widget
    self.object.param.update(**{p_name: change.new})
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameterized.py", line 2319, in update
    restore = dict(self_._update(arg, **kwargs))
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameterized.py", line 2345, in _update
    setattr(self_or_cls, k, v)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameterized.py", line 528, in _f
    instance_param.__set__(obj, val)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameterized.py", line 530, in _f
    return f(self, obj, val)
           ^^^^^^^^^^^^^^^^^
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameters.py", line 543, in __set__
    super().__set__(obj,val)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameterized.py", line 530, in _f
    return f(self, obj, val)
           ^^^^^^^^^^^^^^^^^
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameterized.py", line 1498, in __set__
    self._validate(val)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameters.py", line 828, in _validate
    self._validate_value(val, self.allow_None)
  File "/Users/dj/code/panel-thread-repro/venv/lib/python3.11/site-packages/param/parameters.py", line 811, in _validate_value
    raise ValueError(
ValueError: Number parameter 'PositionView.position' only takes numeric values, not <class 'str'>.```

Screenshots or screencasts of the bug in action

panel.bug.screencast.mov

• I may be interested in making a pull request to address this