The problem is that .worksheets always return Worksheet and does not ever return _ReadOnlyWorksheet: https://github.com/python/typeshed/blob/main/stubs/openpyxl/openpyxl/workbook/workbook.pyi#L61

And load_workbook always returns Workbook

So, we don't have enough typing information right now to make this work.

Ideas?

Could we use @overload and Literal for the load_workbook() function, where if read_only is the literal True then we instead return some kind of new ReadOnlyWorkbook type which itself returns _ReadOnlyWorksheet from .worksheets (and then ReadOnlyCell | EmptyCell for it's rows). Is that a feasible idea?

https://mypy.readthedocs.io/en/stable/literal_types.html#literal-types

Yes, seems ok to me :)
Do you want to work on this?

Do you want to work on this?

@sobolevn I'll aim to take a look 👍🏼

Do note that this problem goes one step further: ReadOnlyWorksheet cell methods come from Worksheet, so you'd have to specialize ReadOnlyWorksheet's cell-related method types on top of making Workbook generic.