(Idea) Preserve application state when in debug mode.

[rmarren1]

The reason https://github.com/plotly/dash-renderer/pull/60 was added in dash-renderer is that application state is maintained in the front-end Redux store and not in the back-end. This makes sense for production applications since we want Dash servers to be completely stateless, but I don't see why this has to be true when in development mode. As long as we carefully code it as to use the state only to do validation and debugging and not affect actual app execution, and figure out a way to clear this state on page refresh, it should be be fine.

Some cool things we can do if app.layout is constantly updated:

• Implement https://github.com/plotly/dash-renderer/pull/60 in the back-end throwing a specific Dash Exception. We can effectively remove 'suppress_callback_exceptions' and do this step dynamically
• Print a 'diff' of the state tree after each callback
• Make bug reports super simple,
• Make it super easy to create test fixtures for Dash apps

This would be relatively simply to implement too.

[valentijnnieman]

That's interesting! I can see that printing a diff of the state tree would be super helpful in debug mode, and open the way to more developed developer's tools. I'm not completely familiar with dash-renderer yet, but wouldn't this be possible to do there as well? Maybe it makes more sense to implement that on the front end after all?

[gvwilson]

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