Use button to trigger action in Panel with Parameterized Class and when button action is finished have another dependency updated (Holoviz)

Question

I am building a dashboard with Panel Holoviz using a Parameterized Class .

In this Class I would like a button that, when pushed starts training a model, and when the model is finished training, it needs to show a graph based on that model.

How do I build such dependencies in Panel using a Class?

Answer 1

The example below shows how when the button is pushed, it triggers 'button', which triggers method train_model(), which when finished, triggers method update_graph().
The key lies in the lambda x: x.param.trigger('button') and @param.depends('button', watch=True):

import numpy as np
import pandas as pd
import holoviews as hv
import param
import panel as pn
hv.extension('bokeh')

class ActionExample(param.Parameterized):
       
    # create a button that when pushed triggers 'button'
    button = param.Action(lambda x: x.param.trigger('button'), label='Start training model!')
      
    model_trained = None
    
    # method keeps on watching whether button is triggered
    @param.depends('button', watch=True)
    def train_model(self):
        self.model_df = pd.DataFrame(np.random.normal(size=[50, 2]), columns=['col1', 'col2'])
        self.model_trained = True

    # method is watching whether model_trained is updated
    @param.depends('model_trained', watch=True)
    def update_graph(self):
        if self.model_trained:
            return hv.Points(self.model_df)
        else:
            return "Model not trained yet"

action_example = ActionExample()

pn.Row(action_example.param, action_example.update_graph)

Helpful documentation on the Action button:
https://panel.pyviz.org/gallery/param/action_button.html#gallery-action-button

Other helpful example of Action param:
https://github.com/pyviz/panel/issues/239

BEFORE pushing button:

AFTER pushing button: