[join-] concat: store src sheet names in a hidden col

[midichef]

Here's a prototype of a feature for convenience. When joining sheets with concat, add a hidden column with the name of the source sheet.

Should we do the same for full, append, and merge?

[saulpw]

Isn't there already a hidden column origin_sheet on ConcatSheet with the source sheet in it?

[frosencrantz]

It looks like append has it, and I use it all the time with append ConcatSheets. It looks like full and merge sheets do not.

[midichef]

Okay, so append gets a origin_sheet, but concat, full, merge and diff don't. I think that column should be added to all of them.

Also, I just noticed that joining a sheet with full or diff fails when there are no key columns on any sheet:

visidata/visidata/features/join.py

Lines 75 to 76 in 57ff6af

	if 0 in nkeys or len(nkeys) != 1:
	vd.fail(f'all sheets must have the same number of key columns')

I'd like that join to succeed when all the joined sheets have 0 key columns.

Should I implement both of these?

[saulpw]

Thanks, @midichef, an implementation would be appreciated. Ideally it would work like it does for concat, with the sheet object being stored in the underlying row data structure, and a hidden column providing the sheet object itself (which will display as its name).

I'm not sure what origin_sheet would mean for those other join types (like full), which pull data from multiple sheets. Do you think it should be origin_sheets in those cases, and expose an array of all sheets the data came from?