How do I make a QTreeView always sort items of a certain category first?

Question

I would like to display "folders" and "files" in a QTreeView. Folders are meant to be able to contain files, and due to this relationship I wish for folder items to be displayed above file items in the tree view. The view should be sortable. How do I make sure that folder items are displayed above file items at all times in the tree view?

The below code provides an example of a tree view with folder and file items:

from PyQt5.QtCore import *
from PyQt5.QtWidgets import *
from PyQt5.QtGui import *


def _create_item(text, is_folder):
    item = QStandardItem(text)
    item.setData(is_folder, Qt.UserRole)
    return item


def _folder_row(name, date):
    return [_create_item(text, True) for text in (name, date)]


def _file_row(name, date):
    return [_create_item(text, False) for text in (name, date)]


class _Window(QMainWindow):
    def __init__(self):
        super().__init__()

        widget = QWidget()
        self.__view = QTreeView()
        layout = QVBoxLayout(widget)
        layout.addWidget(self.__view)
        self.setCentralWidget(widget)

        model = QStandardItemModel()
        model.appendRow(_file_row('File #1', '01.09.2014'))
        model.appendRow(_folder_row('Folder #1', '01.09.2014'))
        model.appendRow(_folder_row('Folder #2', '02.09.2014'))
        model.appendRow(_file_row('File #2', '03.09.2014'))
        model.setHorizontalHeaderLabels(['Name', 'Date'])
        self.__view.setModel(model)
        self.__view.setSortingEnabled(True)


app = QApplication([])
w = _Window()
w.show()
app.exec_()

Answer 1

A solution is to wrap the model in a QSortFilterProxyModel, and reimplement the proxy's lessThan method to make it so that folder items are always placed before file items:

from PyQt5.QtCore import *
from PyQt5.QtWidgets import *
from PyQt5.QtGui import *


def _create_item(text, is_folder):
    item = QStandardItem(text)
    item.setData(is_folder, Qt.UserRole)
    return item


def _folder_row(name, date):
    return [_create_item(text, True) for text in (name, date)]


def _file_row(name, date):
    return [_create_item(text, False) for text in (name, date)]


class _SortProxyModel(QSortFilterProxyModel):
    """Sorting proxy model that always places folders on top."""
    def __init__(self, model):
        super().__init__()
        self.setSourceModel(model)

    def lessThan(self, left, right):
        """Perform sorting comparison.

        Since we know the sort order, we can ensure that folders always come first.
        """
        left_is_folder = left.data(Qt.UserRole)
        left_data = left.data(Qt.DisplayRole)
        right_is_folder = right.data(Qt.UserRole)
        right_data = right.data(Qt.DisplayRole)
        sort_order = self.sortOrder()

        if left_is_folder and not right_is_folder:
            result = sort_order == Qt.AscendingOrder
        elif not left_is_folder and right_is_folder:
            result = sort_order != Qt.AscendingOrder
        else:
            result = left_data < right_data
        return result
        

class _Window(QMainWindow):
    def __init__(self):
        super().__init__()

        widget = QWidget()
        self.__view = QTreeView()
        layout = QVBoxLayout(widget)
        layout.addWidget(self.__view)
        self.setCentralWidget(widget)

        model = QStandardItemModel()
        model.appendRow(_file_row('File #1', '01.09.2014'))
        model.appendRow(_folder_row('Folder #1', '01.09.2014'))
        model.appendRow(_folder_row('Folder #2', '02.09.2014'))
        model.appendRow(_file_row('File #2', '03.09.2014'))
        model.setHorizontalHeaderLabels(['Name', 'Date'])
        proxy_model = _SortProxyModel(model)
        self.__view.setModel(proxy_model)
        self.__view.setSortingEnabled(True)


app = QApplication([])
w = _Window()
w.show()
app.exec_()