Multiple inputs into one depend calls

Panel
,
1

Hi all,

apologies for the newbie questions. I have an issue here where I need to track both input1 and input2 but I do not want the main_pane called twice when the pane is rendered, which is the current behavior, but only once for a change in a set of inputs. This is an issue for me because the main_pane will start a callback for a fast refresh, and if my number of inputs grows, there will be a callback created for each input, instead of just one being created for the whole set of inputs. Is there a way to achieve this behavior?

In the code below you can see that if you change any of the inputs, the num_of_calls increments by 2 instead of 1.

Many Thanks
JJ

input_1 = pn.widgets.Select(options=[1, 2, 3, 4], name='Input 1')
input_2 = pn.widgets.Select(options=[1, 2, 3, 4], name='Input 2')

def sidebar():
    return pn.Row(pn.Column(pn.Row(input_1), pn.Row(input_2)))


@pn.depends(input_1, input_2)
def main_pane(input1_val, input2_val):
    global num_of_calls
    num_of_calls += 1
    md = pn.pane.Markdown(f'{input1_val + input2_val} - {num_of_calls}')
    return pn.Row(pn.Column(md))


if __name__.startswith('bokeh'):
    pn.extension()
    t = pn.template.FastGridTemplate(
        site='Panel', title='Test',
        sidebar=sidebar()
    )
    t.main[:1, :2] = main_pane
    t.servable()
2

This should do the trick t.main[:1, :2] = pn.panel(main_pane, lazy=True)

1 Like
3

Cheers @Hoxbro - this worked. Out of curiosity, why is it so? If you donā€™t mind explaining please. Iā€™ve been struggling to sort this out for a day so quite pleased with the approach, but would be nice to know what is different here.

Thanks
JJ

4

Panel by default runs code eagerly and somewhere in the template pn.panel (or similar) is run. By setting pn.panel(..., lazy=True) the template skips running pn.panel and by setting lazy=True runs it lazily.

1 Like
5

cheers. tyvm for explaining.

6

Theres just one problem with this approach. Maybe a bug.

running lazily with this

panel server app.py

causes the session_info object to not track sessions:

{ā€˜totalā€™: 0, ā€˜liveā€™: 0, ā€˜sessionsā€™: OrderedDict()}

whereas if you tack on the admin app with:

panel server app.py --admin

the session object gets populated:

{ā€˜totalā€™: 1, ā€˜liveā€™: 0, ā€˜sessionsā€™: OrderedDict([ā€¦]}

Do you think this is a bug that needs to be logged.

TIA
JJ

7

I donā€™t follow what you are saying.

Why is this a problem? And is the session you are seeing the admin page?

8

If this app below is run with

panel serve

line 16 prints

{ā€˜totalā€™: 0, ā€˜liveā€™: 0, ā€˜sessionsā€™: OrderedDict()}

no matter how many sessions are open. Iā€™m presuming this is a problem if a developer intends to track user session objects programmatically.

import panel as pn

input_1 = pn.widgets.Select(options=[1, 2, 3, 4], name='Input 1')
input_2 = pn.widgets.Select(options=[1, 2, 3, 4], name='Input 2')
num_of_calls = 0


def sidebar():
    return pn.Row(pn.Column(pn.Row(input_1), pn.Row(input_2)))


@pn.depends(input_1, input_2)
def main_pane(input1_val, input2_val):
    global num_of_calls
    num_of_calls += 1
    print(pn.state.session_info)
    md = pn.pane.Markdown(f'{input1_val + input2_val} - {num_of_calls}')
    return pn.Row(pn.Column(md))


if __name__.startswith('bokeh'):
    pn.extension()
    t = pn.template.FastGridTemplate(
        site='Panel', title='Test',
        sidebar=sidebar()
    )
    t.main[:1, :2] = pn.panel(main_pane, lazy=True)
    t.servable()
9

For me, it also happens with lazy=False and not using --admin in panel serve, though Iā€™m running on the master branch.

So it only seems to work when I have --admin turned on. Honestly, I donā€™t know if this is a design decision or a bug. It makes sense that you only track users when running in ā€œadmin modeā€.

1 Like
10

understood. tyvm for taking the time.

Best
JJ

1 Like
11

raised Empty session_info when rendering with lazy=True Ā· Issue #3816 Ā· holoviz/panel Ā· GitHub for review by devs.