Feature Request #2802

[justinzhu17]

Attempt to implement feature request #2802

Start with a description of this PR. Then edit the list below to the items that make sense for your PR scope, and check off the boxes as you go!

Contributor Checklist

• I have broken down my PR scope into the following TODO tasks
  • Implement a way to delete data from heatmaps within Patch()
  • Create local tests to check for bugs
• I have run the tests locally and they passed. (refer to testing section in contributing)
• I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

optionals

• I have added entry in the CHANGELOG.md
• If this PR needs a follow-up in dash docs, community thread, I have mentioned the relevant URLS as follows
  • this GitHub #PR number updates the dash docs
  • here is the show and tell thread in Plotly Dash community

[T4rk1n]

For #2802 we need something that will handle not knowing the length of the array to be patched.
The implementation would be on the frontend I think in this function you would have to get all the paths if you encounter a marked case.

dash/dash/dash-renderer/src/actions/patch.ts

Lines 36 to 45 in 4a9f09a

	function getLocationPath(location: LocationIndex[], obj: any) {
	const current = [];
	for (let i = 0; i < location.length; i++) {
	const value = getLocationIndex(location[i], path(current, obj));
	current.push(value);
	}
	return current;
	}

Then on the Python side there would be an object representing FOREACH operations of an array

class _ForEach:
    def to_plotly_json():
        return {"_dash_patch_index": "FOREACH"}

# singleton then imported in dash for easy access.
FOREACH = _ForEach()

Then you would use in callback like this:

from dash import Dash, Input, html, Patch, FOREACH

def callback(_):
    fig = Patch()
    del fig['data'][0]['z'][FOREACH][0]
    return p