[Feature Request] Add `allow_optional` for `Output`

[antonymilne]

allow_optional is super useful for Input and State. Pretty please would it be possible to add it to Output too? 🙏

I would have liked to use this a few times before in callbacks where I use optional inputs, but now with devtool.hook it seems impossible to achieve some very useful functionality without having it.

Currently it seems like the components added by devtool.hook should be addressable in callbacks. There's no docs on this, but it's explicitly tested. Clearly this needs allow_optional since the component is only present when debug=True.

So at the moment it's possible to use components added in devtools as callback inputs/states but not as outputs. This feels like it's missing some very useful functionality that would be enabled if we could set allow_optional on Output.

How would an Output(allow_optional=True) work in practice? I guess the callback function would still return a value, but if the output doesn't exist it just doesn't raise an error for a non-existent Dash component.

[antonymilne]

Oh wait, is this achieved by #3397? 🤔 But then it's still a bit weird if you can set optional at the level of Input, State and callback but not Output.

[T4rk1n]

The outputs are not transfered as an object, it was easier to just put optional for all dependencies.

[gvwilson]

@antonymilne does @T4rk1n 's response answer your question? If so, can we close this issue?

[antonymilne]

Ummm not really, sorry 😬 What does "The outputs are not transfered as an object" mean exactly? 🤔 And does this mean that callback(optional=True) effectively sets allow_optional=True for State, Input and Output even though you can't individually set Output(allow_optional=True)?

[T4rk1n]

What does "The outputs are not transfered as an object"

It's just the output are not sent to the backend like the inputs and state in a dictionary but instead they are concatenated strings, that means they can't transfer metadata for the output unless we change the format.

[antonymilne]

ok, got it, thank you! So what exactly is the effect of optional=True for a callback? Does it behave as if all State, Input and Output have allow_optional=True?

[T4rk1n]

Does it behave as if all State, Input and Output have allow_optional=True?

Yes, the callbacks that target components in the devtool are not in the layout, they would all need to be marked as allow_optional. With the optional on the callback level it is more convenient.

[antonymilne]

Got it, thank you!