class CheckableItemsModel(QStandardItemModel):
    checkStateChanged = pyqtSignal()
    def __init__(self, parent=None):
        super(CheckableItemsModel, self).__init__(parent)
    def flags(self, index):
        return super(CheckableItemsModel, self).flags(index) | Qt.ItemIsUserCheckable
    def data(self, index, role=Qt.DisplayRole):
        value = super(CheckableItemsModel, self).data(index, role)
        if index.isValid() and role == Qt.CheckStateRole and value is None:
            value = Qt.Unchecked
        return value
    def setData(self, index, value, role=Qt.EditRole):
        ok = super(CheckableItemsModel, self).setData(index, value, role)
        if ok and role == Qt.CheckStateRole:
            self.checkStateChanged.emit()
        return ok
class CheckComboBox(QComboBox):
    checkedItemsChanged = pyqtSignal(list)
    def __init__(self, parent=None):
        super(CheckComboBox, self).__init__(parent)
        # workaround for Mac and GTK to show checkboxes
        self.setStyleSheet('QComboBox { combobox-popup: 1px }')
        self.defaultText = ''
        self.separator = ','
        self.containerPressed = False
        self.checkableModel = CheckableItemsModel(self)
        self.setModel(self.checkableModel)
        self.model().checkStateChanged.connect(self.updateCheckedItems)
        self.model().rowsInserted.connect(self.updateCheckedItems)
        self.model().rowsRemoved.connect(self.updateCheckedItems)
        self.activated.connect(self.toggleCheckState)
    def itemCheckState(self, index):
        return self.itemData(index, Qt.CheckStateRole)
    def setItemCheckState(self, index, state):
        self.setItemData(index, state, Qt.CheckStateRole)
    def checkedItems(self):
        items = list()
        if self.model():
            index = self.model().index(0, self.modelColumn(), self.rootModelIndex())
            indexes = self.model().match(index, Qt.CheckStateRole, Qt.Checked, -1, Qt.MatchExactly)
            for i in indexes:
                items.append(index.data())
        return items
    def setCheckedItems(self, items):
        for i in items:
            index = self.findText(i)
            self.setItemCheckState(index, Qt.Checked if index != -1 else Qt.Unchecked)
    def updateCheckedItems(self):
        items = self.checkedItems()
        if len(items) == 0:
            self.setEditText(self.defaultText)
        else:
            self.setEditText(self.separator.join(items))
        self.checkedItemsChanged.emit(items)
    def toggleCheckState(self, index):
        value = self.itemData(index, Qt.CheckStateRole)
        if value is not None:
            self.setItemData(index, Qt.Checked if value == Qt.Unchecked else Qt.Unchecked, Qt.CheckStateRole)

 from PyQt4 import QtGui, QtCore
import sys, os
class CheckableComboBox(QtGui.QComboBox):
    def __init__(self):
        super(CheckableComboBox, self).__init__()
        self.view().pressed.connect(self.handleItemPressed)
        self.setModel(QtGui.QStandardItemModel(self))
    def hidePopup(self):  #переопределяем hidePopup
        if not self.view().underMouse():      ##скрывать выпадающий список только если курсор не над списком
            QtGui.QComboBox.hidePopup(self)
    def handleItemPressed(self, index):
        item = self.model().itemFromIndex(index)
        if item.checkState() == QtCore.Qt.Checked:
            item.setCheckState(QtCore.Qt.Unchecked)
        else:
            item.setCheckState(QtCore.Qt.Checked)
class Dialog_01(QtGui.QMainWindow):
    def __init__(self):
        super(QtGui.QMainWindow,self).__init__()
        myQWidget = QtGui.QWidget()
        myBoxLayout = QtGui.QVBoxLayout()
        myQWidget.setLayout(myBoxLayout)
        self.setCentralWidget(myQWidget)
        self.ComboBox = CheckableComboBox()
        for i in range(3):
            self.ComboBox.addItem("Combobox Item " + str(i))
            item = self.ComboBox.model().item(i, 0)
            item.setCheckState(QtCore.Qt.Unchecked)
        myBoxLayout.addWidget(self.ComboBox)
if __name__ == '__main__':
    app = QtGui.QApplication(sys.argv)
    dialog_1 = Dialog_01()
    dialog_1.show()
    dialog_1.resize(480,320)
    sys.exit(app.exec_())