TextInput stuck when running heavy calculations on server

[MHC03]

When I am trying to run heavy calculations and change the value of the TextInput beforehand, the value change somehow seems to be stuck. I have prepared a small example:

pn.extension()

def heavy_calculation():
    epos_indexed = epos[0:10_000_000]
    cropped_index = np.where((epos_indexed['dx'] >= -5) & (epos_indexed['dx'] <= 10) 
                              & (epos_indexed['dy'] >= -10) & epos_indexed['dy'] <= 15)[0]
    # print(list(cropped_index))
    cropped_epos = epos.iloc[list(cropped_index)]

class Test:
    
    def __init__(self):
        self.button = pn.widgets.Button(name='Click Me!')
        self.txt = pn.widgets.TextInput(name='Misused as Log:')
        self.button.param.watch(self._button_click, 'clicks')
        
    def _button_click(self, clicks):
        self.txt.value = 'Start heavy calculation...'
        heavy_calculation()
        self.txt.value = 'Heavy calculation finished.'
    
    def view(self):
        return pn.Column(self.button, self.txt)

Test().view()

When I run this on a jupyter notebook, there is no problem:

But when I deploy it using panel serve --show, the first message on the TextInput widget does not appear:

For deploying I have downloaded the notebook as a python file and edited Test().view() to Test().view().server_doc().

epos is just a pandas DataFrame containing 36,000,000 elements.
I am not sure, whether this is a panel or bokeh problem.

[philippjfr]

Thanks for the report. I'll look into but as a quick aside:

For deploying I have downloaded the notebook as a python file and edited Test().show() to Test().show().server_doc().

This shouldn't be necessary, you can have the "app" work both in the notebook and on bokeh server by using Test().show().servable(). That will render in a notebook and work when you use panel serve notebook.ipynb --show.

Also when you want to test bokeh server from a notebook you could do Test().show().show()

[philippjfr]

Okay, so this is a pretty central part of how bokeh server works, changing a parameter value will schedule a so called next_tick_callback, which won't be processed while the "heavy_calculation" is being processed. Ordinarily you could also schedule the calculation as a next_tick_callback and we could consider exposing that mechanism in an easy way. However it would of course be a lot nicer if we could somehow make the naive approach work, I'll have to read a bit more about tornado and bokeh server to see if that's feasible.

[MHC03]

Thank you very much for looking into all of the issues. For me personally either way is fine, if it is too hard to make it work the naive way, I can try with the next_tick_callback. (But I have to look into how this works)

[philippjfr]

@MHC03 So there's probably nothing we can do automatically, however you can do it by using the tornado IOLoop, in your example you'd do something like:

    def _button_click(self, clicks):
        self.txt.value = 'Start heavy calculation...'
        ioloop = tornado.ioloop.IOLoop.current()
        ioloop.add_callback(self._heavy_calculation)
        
    def _heavy_calculation(self):
        heavy_calculation()
        self.txt.value = 'Heavy calculation finished.'

[MHC03]

For me the above approach does not fully work, the second change to the textinput value does not occur. It is stuck with 'Start heavy calculation...'. If I change the code to following it works in a server but not in a Jupyter Notebook as intended:

def _button_click(self, clicks):
        self.txt.value = 'Start heavy calculation...'
        ioloop = tornado.ioloop.IOLoop.current()
        ioloop.add_callback(heavy_calculation)
        self.txt.value = 'Heavy calculation finished.'

With this I get the expected behaviour in a server, in a Jupyter Notebook it directly jumps to the last line without waiting for the callback to finish. If I have several heavy calculations it gets a bit more complicated through this method.

[jbednar]

This code would be a bit less confusing if you changed your show() method to view(), to avoid confusion with Panel's .show() method.

[philippjfr]

For me the above approach does not fully work, the second change to the textinput value does not occur. It is stuck with 'Start heavy calculation...'.

Yeah, it's not quite clear to me what's going on here, it's definitely not working right in this case but I've used it successfully in other examples.

[MHC03]

This code would be a bit less confusing if you changed your show() method to view(), to avoid confusion with Panel's .show() method.

Changed it, maybe @philippjfr can also edit his comment.

Yeah, it's not quite clear to me what's going on here, it's definitely not working right in this case but I've used it successfully in other examples.

I will try some workaround with the ioloop callback for now. Thanks a lot for looking into this stuff.

[philippjfr]

Thanks a lot for looking into this stuff.

No problem, I'm encountering the same issues so this is fairly high priority. Still searching for a better solution, but my knowledge of tornado is lacking a bit.