'Ensure only one variable can be dropped onto the view.'

def dropMimeData(self, mime, action, row, column, parent):

    vars = mime.property('_items')
    if (vars is None):
        return False
    if (action == Qt.IgnoreAction):
        return True
    return VariableListModel.dropMimeData(self, mime, action, row, column, parent)
