panel.Param does not support changing objects of ObjectSelector dynamically

[MarcSkovMadsen]

Panel: 0.9.5
bokeh: 2.0.2

I was trying to help a user on Discourse here https://discourse.holoviz.org/t/based-on-a-select-widget-update-a-second-select-widget-then-how-to-link-the-latter-to-a-reactive-plot/917.

I could develop an answer to his question but I believe there is a bug. I had to implement a hack/ work around to get it working.

import numpy as np
import matplotlib.pyplot as plt
from matplotlib.figure import Figure
import colorcet as cc
import panel as pn
import param

COLLECTIONS = ["matplotlib", "colorcet"]

MATPLOTLIB_COLORMAPS = sorted(
    [
        "viridis",
        "plasma",
    ],
    key=str.casefold,
)

COLORSET_COLORMAPS = sorted(
    [
        "bgy",
        "bkr",
        "bgyw",
    ]
)
COLOR_MAPS = {"matplotlib": MATPLOTLIB_COLORMAPS, "colorcet": COLORSET_COLORMAPS}


class ColorApp(param.Parameterized):
    collection = param.ObjectSelector(default="matplotlib", objects=COLLECTIONS)
    color_map = param.ObjectSelector(default="viridis", objects=MATPLOTLIB_COLORMAPS)
    use_hack = param.Boolean(default=False)

    settings_pane = param.Parameter()
    plot_pane = param.Parameter()
    view = param.Parameter()

    def __init__(self, **params):
        params["settings_pane"] = pn.Param(
            self, parameters=["collection", "color_map", "use_hack"], show_name=False, width=300, background="grey"
        )
        params["view"] = pn.Row(sizing_mode="stretch_width")

        super().__init__(**params)

        self.view[:] = [self.settings_pane]

    @param.depends("collection", watch=True)
    def _update_color_map_parameters(self):
        print(f"update to {self.collection}")
        objects = COLOR_MAPS[self.collection]
        self.param.color_map.objects = objects

        default = objects[0]
        if self.param.color_map.default not in objects:
            self.param.color_map.default = default
        if self.color_map not in objects:
            self.color_map = default

        # hack to get drowdowns and plot to update ???
        # I should file an issue on Github
        if self.use_hack:
            self.settings_pane.object = self
            self.settings_pane.parameters = ["collection", "color_map", "use_hack"]


if __name__.startswith("bokeh"):
    ColorApp().view.servable()
else:
    ColorApp().view.show(port=5007)

As you can see in the .gif video above the second select widget does not update in response to a change in selection of the first select widget. But if I enable the hack it does.

[MarcSkovMadsen]

The hack does not work nicely as there is a glitch when the panel.Param pane updates. If there is a more efficient hack/ workaround to trigger the update, that would also help me.

[Hoxbro]

Not exactly the way you do it, but what about the following code?

import panel as pn
import param

COLLECTIONS = ["matplotlib", "colorcet"]

MATPLOTLIB_COLORMAPS = sorted(["viridis", "plasma"], key=str.casefold)

COLORSET_COLORMAPS = sorted(["bgy", "bkr", "bgyw"])
COLOR_MAPS = {"matplotlib": MATPLOTLIB_COLORMAPS, "colorcet": COLORSET_COLORMAPS}


class ColorApp(param.Parameterized):
    def __init__(self, **params):
        self.collection = pn.widgets.Select(name="Collection", options=COLLECTIONS)

        self.color_map = pn.widgets.Select(
            name="Color map", options=MATPLOTLIB_COLORMAPS
        )
        self.setting_pane = pn.Column(
            self.collection, self.color_map, width=300, background="grey",
        )

        super().__init__(**params)

    @pn.depends("collection.value", watch=True)
    def _update_color_map_parameters(self):
        print(f"update to {self.collection.value}")
        objects = COLOR_MAPS[self.collection.value]
        self.color_map.options = objects
        self.color_map.value = self.color_map.options[0]


ColorApp().setting_pane.servable()

May I ask what the advantages is of using param objects instead of panel objects?

[MarcSkovMadsen]

Thanks @hoxbro

I very much like focusing on the business problem, i.e. using Param to model that. And only in the end defining layout, styling and widgets.

I think it is more flexible, scales better and is more maintainable.

But maybe its just a subjective thing.

[Hoxbro]

Thank you for your explanation, I will properly try to see how it works next time I do something similar. :)

I found out you can avoid the glitch by changing the precedence to -1 in param.parameters, which always hides the parameters.

settings_pane = param.Parameter(precedence=-1)
plot_pane = param.Parameter(precedence=-1)
view = param.Parameter(precedence=-1)

The glitch you see is because it shows all parameters and then hides them except collection, color_map and use_hack. With this the second line of your hack is properly not needed.

I also found that the Google Map Viewer example in the documentation seems to have the same problem. Maybe a git bisect could check when the example stopped working.

[MarcSkovMadsen]

That’s an interesting finding @Hoxbro . I’m wondering if in general there would be a benefit for setting precedence to -1 for reactive panel components and holoviews plots I don’t expect to show via Param?

[Hoxbro]

I would properly only set precedence to -1 if it is absolute needed to not create any confusion.

I have finally got the example to work without doing a hack by changing the __init__ to the following:

    def __init__(self, **params):
        super().__init__(**params)
        self.settings_pane = pn.Param(
            self, parameters=["collection", "color_map", "use_hack"], show_name=False, width=300, background="grey"
        )
        self.view = pn.Row(sizing_mode="stretch_width")
        self.view[:] = [self.settings_pane]

[MarcSkovMadsen]

@Hoxbro . So the solution is to use Param after the super init?

I think I have experienced problems before actually using Param before super init.

@philippjfr . Is there an explanation for that? Is it a bug or did I do something not supported?

[philippjfr]

You should always initialize parameters via the super before doing anything with them. If you have initialized with super and it still errors it's definitely a bug though.