Improved callback_context

[AnnMarieW]

closes # #1818
#1054

See also: Community Forum Discussion

This PR adds features to dash.callback_context to make it easier to use:

• simplifies the syntax for determining which input triggered a callback.
• adds ctx as an alias for dash.callback_context
• adds ability access the ctx dictionary with the dot notation
• simplifies the callback function signature when there are many inputs.
• makes the dict id easier to access in callbacks using pattern matching.

Determining which input triggered a callback

The new features will replace boilerplate such as:

button_id = dash.callback_context.triggered[0]['prop_id'].split('.')[0]

👉 See the Announcement in the forum for details and examples.

[alexcjohnson]

Does this need to be a separate instance? Can we just say ctx = callback_context after all the imports, and remove the extra instantiation?

[alexcjohnson]

As just discussed on slack, this looks great! Aside from my comment above, this just needs:

• a changelog entry
• resolving conflicts
• flattening the docstring indentation here and here

[AnnMarieW]

@alexcjohnson I completed the following:

As just discussed on slack, this looks great! Aside from my comment above, this just needs:

• a changelog entry
• resolving conflicts
• flattening the docstring indentation here and here

Also, I removed the extra callback_context instance per your comment.
One test in test-39 fails, but it's unrelated to any changes in this PR -- I'm not sure what's wrong.

[alexcjohnson]

Fantastic! 💃

[MattesR]

IDK whether this is the right spot. I'd just want to mention that there's a typo in this example:
example app using the new ctx.triggered_ids to get the index of the button that triggered the callback:

it says prop_id=ctx.triggered_ids.first() but it should be prop_id=ctx.triggered_id.first(). Found out when trying to update my code with the new ctx.

ctx has the attributes 'triggered_id', and 'triggered_prop_ids' according to dir(ctx)

[AnnMarieW]

@MattesR Thanks for your comment. I think the examples in the initial write-up for this PR were confusing because the names changed during review. For example triggered_ids was renamed to triggered_prop_ids. Then a new attribute was added: ctx.triggered_id, which is the component id of the the Input that triggered the callback.

I just updated the comment to remove all the examples here and point people to the ones in the announcement on the community forum.