First confirm which of the two you want. Let's guess it's the first, you want it stored as a number and just want to control the output format. Then the simplest is to use QStyledItemDelegate::displayText().

To make this operate only on a given column. Either create your own class DoubleStyledItemDelegate: public QStyledItemDelegate and install on just that column via QAbstractItemView::setItemDelegateForColumn(). Or in a common QStyledItemDelegate you would override QStyledItemDelegate::paint(), which is passed the model index to identify the column, and produce the desired text to print there (example at, say, https://stackoverflow.com/questions/34729858/override-text-in-qstyleditemdelegate-for-qtreeview). If you can, the first alternative seems preferable and re-usable.

My question was if it's possble to tweak what's displayed by the QTableWidget for one column (i.e. by changing the DisplayRole) without having to implement the whole thing.

I can't use a QSortFilterProxyModel, as setting a model is private for a QTableWidget. I can't use QStyledItemDelegate::displayText either, because there's no way to see from which column the data comes …

Edit:

Oh, I cross-posted or you cross-edited. Well, as said, I can't use QStyledItemDelegate::displayText. Well, storing the actual text would be a possibility, but I would have to process the text later on, to get the double value back, because I use the data entered later …

The problem is that if one e.g. enters "5,00 €" in the spin box, I still get "5" after pressing enter, or "5,5" for "5,50 €".

It's a double spin box for entering a floating point number. 5,00 == 5 and 5,50 == 5,5 as numbers. Where exactly are you seeing the text you are quoting?

Do you mean when the editing spin box has gone, the value the user entered has been stored as data and the QTableWidget is displaying that as uneditable text?

If this the case then you have two choices:

• If you want that column to always be shown as, say, 2 decimal places with a euro symbol you can store it as a number like QDoubleSpiinBox::value() gives you, and have the QStyledItemDelegate output it like that always (see displayText()). What the user actually typed is ignored.

• If you want that column to show what the user actually typed into the spin box, not just its value converted to a double, so you keep e.g. how many trailing 0s were typed or whether a currency symbol was entered, then you would have to store the result not as a number but as text. For that the base class has QAbstractSpinBox::text().