When callback exceptions are supressed, if an `Input` or `State` references an ID that doesn't exist at callback trigger, then an front-end exception is thrown

[chriddyp]

[rmarren1]

More details:

when app.config['suppress_callback_exceptions'] = False, dash validates all callbacks when the application is launched and throws a NonExistantIdException here if the initial state of the layout does not include an id used in either the Input, Output, or State of a callback.

E.g.,

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

app = dash.Dash()
app.layout = html.Div([
    html.Button(id='button', children='click'),
    html.P(id='output')
])

@app.callback(Output('output', 'children'),
              [Input('button', 'n_clicks')],
              [State('buttoN', 'n_clicks')])
def put_n_clicks(n_clicks, n_clicks_state):
    if n_clicks:
        return n_clicks
    return 0

app.run_server(debug=True)

-->

dash.exceptions.NonExistantIdException: 
Attempting to assign a callback to the
component with the id "buttoN" but no
components with id "buttoN" exist in the
app's layout.


Here is a list of IDs in layout:
['button', 'output']

This exception would be raised if a user includes a callback using an id of a component that does not exist in the initial state, but is dynamically generated by other callbacks.

For example, an exception is raised in this example:

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

app = dash.Dash()
app.layout = html.Div([
    html.Button(id='button', children='click'),
    html.Div(id='output-div-holder'),
    html.Div(id='output-paragraph')
])

@app.callback(Output('output-div-holder', 'children'),
              [Input('button', 'n_clicks')])
def put_n_clicks(n_clicks):
    return html.Div(children=n_clicks, id='output-div') if n_clicks else 'None'

@app.callback(Output('output-paragraph', 'children'),
              [Input('output-div', 'children')])
def put_child(child):
    return child

because the second callback has an Input for the id 'output-div', but that component does not exist in the initial state (it is generated once the user clicks the button).

To fix this, a user may specify app.config['suppress_callback_exceptions'] = True to let their application run without breaking. Adding that line to the above program makes everything work:

-->

Doing this, however, puts you in debugging hell for large applications since any time you include a callback which has 1. an id which will never exist in the application (you mis-spelled it for example) or 2. an id for a component which does exist in the layout at the time that another Input triggers the callback, you will get nothing but an unhelpful front-end exception:

[rmarren1]

It does not look like app.layout updates in sync with the application front-end, which means it is unlikely we can dynamically check this before each callback and throw a back-end exception.

It appears that the exception is thrown from here. I think the quickest patch right now is just to throw a 'nice' exception there telling the user the bad id and the set of currently available ids.

[chriddyp]

It does not look like app.layout updates in sync with the application front-end, which means it is unlikely we can dynamically check this before each callback and throw a back-end exception.
It appears that the exception is thrown from here. I think the quickest patch right now is just to throw a 'nice' exception there telling the user the bad id and the set of currently available ids.

Yeah, a good exception would go a long way for me. Ultimately, I think that our solution will have to be propogating these error messages from the JS context up to the browser context (in dev-mode) with some type of persistent notifications. That's a much larger project that we should probably discuss in a separate issue

[rmarren1]

Sounds good, I'll make a quick fix with a better exception for now

[varunsampat30]

Hello, I am currently working on a Dash project and came across the same issue, and completely understand why suppressing the warning is bad practise. I followed the thread attached at the end of page (A proposed alternative ...) but didn't really understand it. What is the proposed solution?

Thanks in advance!!