QComboBox Multiple Selection with PyQT5

Question

I'm making the gui for my QGIS3 plugin using PyQT5 and QT designer. I have a very long list of items in a QComboBox and I need the user to be able to select one or more items. I know that QComboBox allows only single selection, so I thought that I can use QListWidget. The problem is that the list of selectable items is very long and I'd need a dropdown menu or something similar.

Is there a way to style a QListWidget like a QComboBox or to allow a multiple selection using QComboBox?

EDIT: I added an immage of my GUI. This is a long list and, as you can see, it's the reason why I can't use QListWidget or QTableWidget. A dropdown checkbox group should be perfect and I'm surprised that QT doesn't make this available.

Answer 1

For anyone interested, here is an implementation of a CheckableComboBox.

class CheckableComboBox(QComboBox):

    # Subclass Delegate to increase item height
    class Delegate(QStyledItemDelegate):
        def sizeHint(self, option, index):
            size = super().sizeHint(option, index)
            size.setHeight(20)
            return size

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)

        # Make the combo editable to set a custom text, but readonly
        self.setEditable(True)
        self.lineEdit().setReadOnly(True)
        # Make the lineedit the same color as QPushButton
        palette = qApp.palette()
        palette.setBrush(QPalette.Base, palette.button())
        self.lineEdit().setPalette(palette)

        # Use custom delegate
        self.setItemDelegate(CheckableComboBox.Delegate())

        # Update the text when an item is toggled
        self.model().dataChanged.connect(self.updateText)

        # Hide and show popup when clicking the line edit
        self.lineEdit().installEventFilter(self)
        self.closeOnLineEditClick = False

        # Prevent popup from closing when clicking on an item
        self.view().viewport().installEventFilter(self)

    def resizeEvent(self, event):
        # Recompute text to elide as needed
        self.updateText()
        super().resizeEvent(event)

    def eventFilter(self, object, event):

        if object == self.lineEdit():
            if event.type() == QEvent.MouseButtonRelease:
                if self.closeOnLineEditClick:
                    self.hidePopup()
                else:
                    self.showPopup()
                return True
            return False

        if object == self.view().viewport():
            if event.type() == QEvent.MouseButtonRelease:
                index = self.view().indexAt(event.pos())
                item = self.model().item(index.row())

                if item.checkState() == Qt.Checked:
                    item.setCheckState(Qt.Unchecked)
                else:
                    item.setCheckState(Qt.Checked)
                return True
        return False

    def showPopup(self):
        super().showPopup()
        # When the popup is displayed, a click on the lineedit should close it
        self.closeOnLineEditClick = True

    def hidePopup(self):
        super().hidePopup()
        # Used to prevent immediate reopening when clicking on the lineEdit
        self.startTimer(100)
        # Refresh the display text when closing
        self.updateText()

    def timerEvent(self, event):
        # After timeout, kill timer, and reenable click on line edit
        self.killTimer(event.timerId())
        self.closeOnLineEditClick = False

    def updateText(self):
        texts = []
        for i in range(self.model().rowCount()):
            if self.model().item(i).checkState() == Qt.Checked:
                texts.append(self.model().item(i).text())
        text = ", ".join(texts)

        # Compute elided text (with "...")
        metrics = QFontMetrics(self.lineEdit().font())
        elidedText = metrics.elidedText(text, Qt.ElideRight, self.lineEdit().width())
        self.lineEdit().setText(elidedText)

    def addItem(self, text, data=None):
        item = QStandardItem()
        item.setText(text)
        if data is None:
            item.setData(text)
        else:
            item.setData(data)
        item.setFlags(Qt.ItemIsEnabled | Qt.ItemIsUserCheckable)
        item.setData(Qt.Unchecked, Qt.CheckStateRole)
        self.model().appendRow(item)

    def addItems(self, texts, datalist=None):
        for i, text in enumerate(texts):
            try:
                data = datalist[i]
            except (TypeError, IndexError):
                data = None
            self.addItem(text, data)

    def currentData(self):
        # Return the list of selected items data
        res = []
        for i in range(self.model().rowCount()):
            if self.model().item(i).checkState() == Qt.Checked:
                res.append(self.model().item(i).data())
        return res

Usage

comunes = ['Ameglia', 'Arcola', 'Bagnone', 'Bolano', 'Carrara', 'Casola', 'Castelnuovo Magra', 
    'Comano, località Crespiano', 'Fivizzano', 'Fivizzano località Pieve S. Paolo', 
    'Fivizzano località Pieve di Viano', 'Fivizzano località Soliera', 'Fosdinovo', 'Genova', 
    'La Spezia', 'Levanto', 'Licciana Nardi', 'Lucca', 'Lusuolo', 'Massa', 'Minucciano', 
    'Montignoso', 'Ortonovo', 'Piazza al sercho', 'Pietrasanta', 'Pignine', 'Pisa',
    'Podenzana', 'Pontremoli', 'Portovenere', 'Santo Stefano di Magra', 'Sarzana',
    'Serravezza', 'Sesta Godano', 'Varese Ligure', 'Vezzano Ligure', 'Zignago' ]
combo = CheckableComboBox()
combo.addItems(comunes)

Result

---

Edit

QGIS defines its own QgsCheckableCombobox, which is unfortunately broken (https://github.com/qgis/QGIS/issues/28658).

It relies on the QCombBox activated signal to toggle on/off its items. The problem is that this signal is emitted when the combobox is opened and there's a single selected item, which is NOT the currentIndex

It's however possible to fix it, albeit in a slightly hackish way

class CheckableComboBox2(QgsCheckableComboBox):

    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.activated.connect(self.afterActivated)   

    def afterActivated(self):
        for i in range ( self.model().rowCount()):
            item = self.model().item(i)  
            if item.checkState() == Qt.Checked:
                self.setCurrentIndex(i)
                self.model().itemCheckStateChanged.emit()
                return