Can't access result updated by long callback

Dash Python
1

I would like to update the attributes of objA with long_callback update_output, show the progress, and access the result with callback access_val.

from dash import Dash, html, Input, Output, callback_context
import diskcache
from dash.long_callback import DiskcacheLongCallbackManager
cache = diskcache.Cache("./cache")
long_callback_manager = DiskcacheLongCallbackManager(cache)

class ObjectB(object):
    
    def __init__(self, val):
        self.val = val

class ObjectA(object):
    
    def __init__(self):

        self.obj = ObjectB(10)

    def update(self):
        self.obj.val2 = 20

    def get_val2(self):
        return self.obj.val2

class DashObject(object):

    def __init__(self, objA):
        
        self.app = Dash(__name__, long_callback_manager=long_callback_manager)
        self.objA = objA

    def run_once(self):
        
        self.app.layout = html.Div([
            html.Button('Submit', id='submit-val', n_clicks=0),
            html.Div(id='container-button-basic',
                    children='Enter a value and press submit'),
            html.Button('Submit2', id='submit-val-2', n_clicks=0),
            html.Div(id='container-button-basic-2',
                    children='Enter a value and press submit'),
            html.Progress(id="progress-bar")
        ])
        self.setup_callbacks()
        self.app.run_server(debug=True)

    def update(self):
        self.objA.update()
    
    def get_val(self):
        return self.objA.get_val2()
    
    def setup_callbacks(self):

        @self.app.long_callback(
            output =    Output('container-button-basic', 'children'),
            inputs =    Input('submit-val', 'n_clicks'),
            progress =  [Output("progress-bar", "children")],
            prevent_initial_call=True,
            
        )
        def update_output(set_progress, n_clicks):
            changed_id = [p['prop_id'] for p in callback_context.triggered][0]
            if 'submit-val' in changed_id:
                self.update()
                set_progress((str(1), str(1)))
                return 'The input value was "{}"'.format(self.get_val())
            
        @self.app.callback(
            Output('container-button-basic-2', 'children'),
            Input('submit-val-2', 'n_clicks'),
            prevent_initial_call=True
        )
        def access_val(n_clicks):
            changed_id = [p['prop_id'] for p in callback_context.triggered][0]
            if 'submit-val-2' in changed_id:
                return 'The input value was "{}"'.format(self.get_val())

if __name__ == "__main__":
    objA = ObjectA()
    Dash_obj = DashObject(objA)
    Dash_obj.run_once()

In the dash app, I first clicked ‘Submit’ to set ObjB.val2 and then clicked ‘Submit2’ to access the value. I ended up getting the following error

Traceback (most recent call last):
File “/home/j.huang/Repos/ssa-code3_3/./pipeline/sensor_tasking/test.py”, line 75, in access_val
return ‘The input value was “{}”’.format(self.get_val())
^^^^^^^^^^^^^^^
File “/home/j.huang/Repos/ssa-code3_3/./pipeline/sensor_tasking/test.py”, line 49, in get_val
return self.objA.get_val2()
^^^^^^^^^^^^^^^^^^^^
File “/home/j.huang/Repos/ssa-code3_3/./pipeline/sensor_tasking/test.py”, line 22, in get_val2
return self.obj.val2
^^^^^^^^^^^^^
AttributeError: ‘ObjectB’ object has no attribute ‘val2’

According to the screen shot below, it seems that val2 once been set can’t be accessed by the subsequent callbacks.

image