openpyxl Worksheet columns is incorrectly typed

[ostr00000]

Property columns should return a generator of tuple[Cell] instead of a single Cell.

This is a very similar issue to #9904, except for column.

[kltws]

Looking at the implementation of Worksheet, particularly in the _clean_merge_range method (called by merge_cells), it seems that the Worksheet._cells dict can contain MergedCell instances in addition to Cell instances. Shouldn't columns (and for that matter, all the other methods like cell, getitem, rows, iter_rows/iter_cols that returns things found in _cells) be typed to reflect this?

[ostr00000]

Type of internal _cells

Yes, the type of Worksheet()._cells is dict[tuple[int, int], Cell | MergedCell], so the return type is indeed Cell | MergedCell.
Current openpyxl implementation for reference: https://foss.heptapod.net/openpyxl/openpyxl/-/blob/branch/3.1/openpyxl/worksheet/worksheet.py#L609

Possible solution

If I understand tips from CONTRIBUTING.md correctly union type should be avoided, because to satisfy type check an explicit if must be written on result from such function.
The preferred solution is to use Any and then annotate the result with expected type (when we know that MergedCell is not used at all) or perform isinstance check.

I see 3 possible solutions:

1. Keep in stub only Cell type - after all MergedCell is not so frequently used.
2. Refactor stub to use Cell | MergedCell - this is real type, but it requires additional check in user code + it is not recommended by typeshed (avoid union return types in https://github.com/python/typeshed/blob/main/CONTRIBUTING.md).
3. Refactor stub to use Any - this is recommended by typeshed, but provides no typing value (without additional, explicit annotation).
4. Refactor stub to use common class for Cell and MergedCell. Fortunately, there is common class StyleableObject. But in that case, a typing.cast would be required (instead of annotating) if somebody would want to use Cell.

There is python/typing#566 that would be handy in this case.
I am ok with no additional type (option 1), so I am not going to create new issue/PR.

Real solution

Of course, a function returning many types is a code smell. According to current usage, most usages want only to get a value:

• increment column until normal cell: while isinstance(cell, MergedCell): column += 1,
• get real cell: cell = self.normalize_merged_cell_link,
• skip processing: warnings.warn(...); continue.

The best solution, in my opinion, is to redesign Worksheet class interface:

• let cell return always Cell object,
• if you are interested whenever the cell is merged, call an additional function.