204 NO CONTENT regression fix

[Marc-Andre-Rivet]

Regression between Dash 0.41 and 0.42

Reproducible with Byron's code below:

import dash
import dash_html_components as html
import dash_core_components as dcc
from multiprocessing import Value
from dash.dependencies import Input, Output
from dash import callback_context, no_update
from dash.exceptions import PreventUpdate
from multiprocessing import Value

initial_input = 'initial input'
initial_output = 'initial output'


app = dash.Dash(__name__)
# app.config['suppress_callback_exceptions'] = True
app.css.config.serve_locally = True
app.scripts.config.serve_locally = True

app.layout = html.Div([
    dcc.Input(id='input', value=initial_input),
    html.Div(initial_output, id='output1'),
    html.Div(initial_output, id='output2'),
])

callback1_count = Value('i', 0)
callback2_count = Value('i', 0)


@app.callback(Output('output1', 'children'), [Input('input', 'value')])
def callback1(value):
    callback1_count.value += 1
    if callback1_count.value > 2:
        return no_update
    raise PreventUpdate("testing callback does not update")


@app.callback(Output('output2', 'children'), [Input('output1', 'children')])
def callback2(value):
    callback2_count.value += 1
    return value

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

Introduced in: https://github.com/plotly/dash-renderer/pull/152/files#diff-4e3037116dbc2bc69a473b7ace3d02abR678 and https://github.com/plotly/dash-renderer/pull/162/files#diff-4e3037116dbc2bc69a473b7ace3d02abR682

[alexcjohnson]

Good catch! Non-blocking, but it would be a bit cleaner as

if (res.status === STATUS.PREVENT_UPDATE) {
    return;
}
throw res;

But I do think this warrants a test case.

[byronz]

But I do think this warrants a test case.

the above sample app from one of integration would be able to catch the issue
but failed to notify as I believe there is a bug in old implementation of check clear console log in selenium. consolidate the IntegrationTest for both dash and dash-renderer with desired_capabilities disclosed it. yes but we would need more customized case later

[alexcjohnson]

consolidate the IntegrationTest for both dash and dash-renderer with desired_capabilities disclosed it.

Ok, if you have a test to 🔒 this elsewhere, we can merge this PR without one. Though it looks like there’s a test already that’s sensitive to this and needs fixing?

[Marc-Andre-Rivet]

Not sure what's going on here but the error was already present when the renderer was bumped to 0.23: https://circleci.com/gh/plotly/dash-renderer/2189?utm_campaign=vcs-integration-link&utm_medium=referral&utm_source=github-build-link

Maybe dash had been updated since the last renderer change and some interaction changed / caused the issue.

[Marc-Andre-Rivet]

Ah! So the failure comes from this change in DCC: https://github.com/plotly/dash-core-components/blame/master/src/components/Input.react.js#L103

As previously discussed, some attention to the builds is still required when bumping versions as some combinations of packages don't get automatically tested yet. In this case dcc was last updated after renderer and the diff was only caught at release.

Updating the test.

[Marc-Andre-Rivet]

The percy diffs are caused by the previous failure in master erasing the baseline screenshots for test_callbacks_generating_children. Approving them.

[alexcjohnson]

💃 thanks for propagating my default input type change to the tests here