Support Previous_Value in fill_nothing and fill_missing

[radeusgd]

Pull Request Description

• Adds Previous_Value to fill_nothing and fill_empty, as requested by Small Libs Tickets for July #7192.

Important Notes

Checklist

Please ensure that the following checklist has been satisfied before submitting the PR:

• The documentation has been updated, if necessary.
• Screenshots/screencasts have been attached, if there are any visual changes. For interactive or animated visual changes, a screencast is preferred.
• All code follows the
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  and
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• All code has been tested:
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[radeusgd]

[radeusgd]

[jdunkerley]

Just a thought (and I know I designed this...) but we could make at/get allow a Column_Ref?

[radeusgd]

We could, but what benefits would that give? at already takes a column name as Text, Column_Ref is useful when we want to distinguish a column name/index from a text/int value, but there is no such risk in at.

And I don't think we ever expect users to manually create a Column_Ref - I think they mostly should rely on just dropdowns.

---

Still, we can allow it to make it more 'robust'. But then I think we need to allow it in all the other places - like select_columns, Aggregate_Column etc. - wherever a Text/Integer column name/index is expected - to be consistent.

IMHO there is no need for that, but happy to be proven wrong