plotly / plotly/dash

chained callback does not used latest value of Store data

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bug P3
Dominant language
Python
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Description

Describe your context

dash                 1.16.3
dash-core-components 1.3.1
dash-html-components 1.0.1
dash-renderer        1.1.2
dash-table           4.4.1

Describe the bug

Chained callback does not use value of data changed in first callback for Store element.
first callback(Output(store, data)Output(text,children)Input(button, n_clicks))
chained_callback(Input(text,children),State(store,data))

See mini example associated

Expected behavior

When first callback is triggered it should first change the data value of store and use that value in the state of chained callback.

Screenshots
When page is loaded:
bug_store_callback_ini

When I click on button1
bug_store_callback

Below is a minimalist example of the code:

import dash
import dash_core_components as dcc
import dash_html_components as html
from dash.dependencies import Input, Output, State
from dash.exceptions import PreventUpdate

app = dash.Dash()
app.layout = html.Div(
    [
        dcc.Store(id="store", storage_type="session", data={'store_content': "ini"}),
        html.Button("b1", id="button1"),
        html.Button("b2", id="button2"),
        html.Div(children="button callback not executed", id="button_output"),
        html.Div(children="chianed callback not executed", id="chained_output"),
    ]
)


@app.callback(
    Output("button_output", "children"),
    Output("store", "data"),
    Input("button1", "n_clicks"),
    Input("button2", "n_clicks"),
    State("store", "data")
)
def button_callback(n1, n2, data):
    ctx = dash.callback_context
    if not ctx.triggered:
        raise PreventUpdate

    button_id = ctx.triggered[0]['prop_id'].split('.')[0]
    if button_id == "button1":
        data['store_content'] = "button1"
        return "button 1 pressed: " + repr(data), data
    elif button_id == "button2":
        data['store_content'] = "button2"
        return "button 2 pressed: " + repr(data), data


@app.callback(
    Output("chained_output", "children"),
    Input("button_output", "children"),
    State("store", "data")
)
def chained_callback(text, data):
    return "chained callback: " + repr(data)


if __name__ == '__main__':
    app.run_server(debug=True)

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running the minimal Dash example to reproduce the callback chain and observe the Store value received by chained_callback. Trace how callback outputs and Store state are propagated between callbacks; done means the chained callback receives the updated Store data after the first callback runs.

Written by the indexing model from the issue text.

Assessment

Tech stack
python
Domain
full-stack
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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