Some Panel widgets are blocked from updating

[kcpevey]

Context

I have a text status bar that I’d like to update during a long compute. If I view the app in JupyterLab, the updates to the status bar happen as python progresses through the code. If I deploy that app, the updates are blocked and are not visible until the python process is complete.

I originally thought this was a bug in Panel so opened it up for discussion here. After discussion, it looks like this has something to do with the way that jhub-apps is deploying the app.

Value and/or benefit

Being able to have a dynamically updated status bar is very useful for long running processes on apps.

Additionally, this may be only one side effect of the root cause and there may be other Panel limitations that we are not yet aware of.

Anything else?

Reproducer:

import panel as pn
import param
import time

pn.extension()

class TestPanel(param.Parameterized):

    def __init__(self, **params):
        self.status_text = pn.widgets.StaticText(name='Status', value='Click "Evaluate"')
        self.evaluate_button = pn.widgets.Button(name='Evaluate', button_type='warning')
        self.evaluate_button.on_click(self.run_evaluate)
        super().__init__(**params)

    def run_evaluate(self, event=None):
        self.status_text.value = 'start'
        time.sleep(3)
        self.status_text.value = 'continuing'
        time.sleep(3)
        self.status_text.value = 'end'

    def panel(self):
        return pn.Column(self.status_text, self.evaluate_button)

app = TestPanel()
app.panel().servable()

[kcpevey]

The same blocking behavior was also seen in Panel Preview. @krassowski pushed updates to bokeh and pyviz_comms to resolve this. We should recheck this once those are released.