implementing callback_fallback

[BSd3v]

implementing callback_fallback on Dash and callback to allow for universal error handling of callbacks.

This manipulates a new variable in _callback GLOBAL_CALLBACK_FALLBACK in order to pass the configuration to all other callbacks from the Dash() registration

see here for a forum post demonstrating this functionality:
https://community.plotly.com/t/error-handling-for-callbacks-and-layouts/83586/2

[ndrezn]

Related: https://community.plotly.com/t/error-handling-for-callbacks-and-layouts/83586/2

[ndrezn]

@BSd3v could you add an example of using this new callback_fallback functionality as implemented here to get a sense of use case/how to document?

[BSd3v]

Here is an example app:

from dash import *
from dash._utils import to_json
import traceback
from dash import ctx
from dash.exceptions import PreventUpdate
import json


def callback_fallback(output=None):
    error_message = "error in callback"

    def decorator(func):
        def wrapper(*args, **kwargs):
            try:
                return func(*args, **kwargs)
            except Exception as e:
                if str(e) == "":
                    raise PreventUpdate
            alertError(f"{error_message}", f"{output}\n {traceback.format_exc()}")
            resp = {
                "app_notification": {
                    "children": json.loads(
                        to_json(
                            notification(
                                "error", "there was an issue, IT has been notified", ctx
                            )
                        )
                    )
                }
            }
            return json.dumps({"multi": True, "response": resp})

        return wrapper

    return decorator

def callback_fallback2(output=None):
    error_message = "error in callback"

    def decorator(func):
        def wrapper(*args, **kwargs):
            try:
                return func(*args, **kwargs)
            except Exception as e:
                if str(e) == "":
                    raise PreventUpdate
            alertError(f"{error_message}", f"{output}\n {traceback.format_exc()}")
            resp = {
                "app_notification": {
                    "children": json.loads(
                        to_json(
                            notification(
                                "error", "I'm sorry Dave, I'm afraid I can't do that", ctx
                            )
                        )
                    )
                }
            }
            return json.dumps({"multi": True, "response": resp})

        return wrapper

    return decorator


app = Dash(
    __name__,
    callback_fallback=callback_fallback,
)

def notification(type, msg, ctx=None):
    if type == 'error':
        return html.Div(
            children=msg + (f' - {ctx.triggered[0]["value"]}' if ctx else ''),
            style={'color': 'red'}
        )
    return ''


def alertError(subject, message):
    print(subject)
    print(message)
    pass

@callback(
    Output("children", "children"),
    Input("click", "n_clicks"),
    State("testChecked", "value"),
    prevent_initial_call=True,
)
def partialFailingCall(n, c):
    if c:
        return rawr
    return f"I ran properly - {n}"

@callback(
    Output("children2", "children"),
    Input("click2", "n_clicks"),
    State("testChecked2", "value"),
    callback_fallback=callback_fallback2,
    prevent_initial_call=True,
)
def partialFailingCall(n, c):
    if not c:
        return rawr
    return f"I ran properly - {n}"


app.layout = html.Div(
                children=[html.Div(id="app_notification"),
                          html.Div([
                            html.Div("When checked, the callback will fail"),
                                  html.Button("test callback", id="click"),
                                  dcc.Checklist([True], id="testChecked"),
                                  html.Div(id="children")
                              ]
                            ),
                          html.Div([
                            html.Div("When not checked, the callback will fail, callback handler is different"),
                                  html.Button("test callback", id="click2"),
                                  dcc.Checklist([True], id="testChecked2"),
                                  html.Div(id="children2")
                              ]
                            )
                          ]
            )

app.run(debug=True)

Notice that in the second callback I am overriding the default callback_fallback in the callback definition.

[ndrezn]

I think this pattern is a bit confusing. We're passing a decorator as an argument: With this API I would expect to pass a function as an argument. I guess in general I feel like there's still some API design to be done here.

What is the ideal way to pass in an error fallback for a Dash callback? It's not immediately obvious to me, but I think we can definitely simplify the pattern from having to pass a decorator. @T4rk1n might have some other comments... but we might want to spend some time in an issue ticket discussing this feature more broadly before starting implementation?

[BSd3v]

alertError(f"{error_message}", f"{output}\n {traceback.format_exc()}")
            resp = {
                "app_notification": {
                    "children": json.loads(
                        to_json(
                            notification(
                                "error", "I'm sorry Dave, I'm afraid I can't do that", ctx
                            )
                        )
                    )
                }
            }
            return json.dumps({"multi": True, "response": resp})

So just pass something like this instead?

[ndrezn]

Maybe, but I'd want to get to something even simpler to use. @T4rk1n to open a ticket describing the feature and we can hone in on a spec from there I think makes the most sense.