[selection] add commands to sort selected rows

[midichef]

This is a proof-of-concept command that sorts only the rows in the selection. @daviewales requested such a feature and I wanted it too.

I recommend using the c79ce4f commit for testing. It uses a temporary workaround for #2266 to allow undo. Otherwise you can't undo the sort. And it uses assertions to detect if rows are lost.

If you think this is worth including, I would appreciate a close inspection of this, @saulpw and @anjakefala , as my first few versions of this caused data corruption.

The practical use of it is a bit limited. Right now you can use sort-selected-asc and -desc to sort by the current column. But if you want to sort by multiple columns you'll have to do it by hand, via exec-python and then
sortSelected(ordering). ordering is a list of (column, boolean) tuples. The boolean is a direction; False means ascending, True means descending. For example, sortSelected([(row1, False), ('title2', True), ('title3'), False)]).

I can imagine a way to reorder rows more flexibly. The user could pull the selection into a new sheet with ", reorder the sheets using keys like [ and z[, or even by sliding rows up and down. And then use a new command to replace the source row selection with the reordered rows. What do you think of that?

[saulpw]

I think these commands should go in their own file, either features/sort_selected.py or experimental/sort_selected.py. I'm leaning towards the latter, because while the code looks right to me, the idea in general makes me nervous--what if the user aborts this thread with Ctrl+C after the sort but before the replacement? It feels like these should be opt-in, use-at-your-own-risk commands, for the time being at least.

I made the other changes to the sort API to support this, and because they feel better in general.

[midichef]

Good points about cancelling being dangerous! I hadn't considered that.

We could bypass this if we could edit a copy of the rows and then assign it to self.rows. But Sheet.sort() says we must not assign to self.rows. What are the specific reasons for that?

visidata/visidata/sort.py

Line 72 in aab3cda

# must not reassign self.rows: use .sort() instead of sorted()

[saulpw]

We could bypass this if we could edit a copy of the rows and then assign it to self.rows. But Sheet.sort() says we must not assign to self.rows. What are the specific reasons for that?

With git log -S reassign I got to 3930ad0:

by not sorting in-place, sorting on columns sheet would not sort columns on source sheet

It's an edge case for sure, but one that I ran across and felt important to make work for internal design consistency.

[midichef]

Oh, I see. Assigning to self.rows on a ColumnsSheet would break the link to the source sheet's columns. It would break sorting in that command. And afterward commands that edit rows, like slide-up, would have no effect on the source sheet. Even cursorRowIndex would give misleading answers that don't correspond to the source sheet columns.

I'll search the git logs more in the future, that note is still helping, 5 years later.