QTreeView свернутые папки по умолчанию

void QTreeView::expandAll()

Разворачивает все расширяемые элементы.

Предупреждение: если модель содержит большое количество элементов, выполнение этой функции может занять некоторое время.

from PyQt5 import QtCore, QtGui, QtWidgets

tree = {
    'child 1-2': [],
    'child 1-3': ["child 1-1-1"],
    'children 2-1': ["..."],
    "parent 2": ["..."],
    "parent 3": [],
    "parent 4": []
}
tree['parent 1'] = {}
tree['parent 1']['child1'] = {}
tree['parent 1']['child1']['child2'] = {}
tree['parent 1']['child1']['child2']['child3'] = ['elem1', 'elem2']
tree['parent 1']['child1']['child3'] = {}


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super(MainWindow, self).__init__(parent)

        self.tree_view = QtWidgets.QTreeView()
        self.setCentralWidget(self.tree_view)

        model = QtGui.QStandardItemModel()
        self.populateTree(tree, model.invisibleRootItem())
        self.tree_view.setModel(model)
        model.setHeaderData(0, QtCore.Qt.Horizontal, 'Записи')
        
#        self.tree_view.expandAll()                                                         # --- !!!
        
        self.tree_view.selectionModel().selectionChanged.connect(self.onSelectionChanged)

    def populateTree(self, children, parent):
        for child in children:
            child_item = QtGui.QStandardItem(child)
            parent.appendRow(child_item)
            if isinstance(children, dict):
                self.populateTree(children[child], child_item)

    def onSelectionChanged(self, *args):
        print(self.tree_view.selectedIndexes())
        for sel in self.tree_view.selectedIndexes():
            val = "/"+sel.data()
            while sel.parent().isValid():
                sel = sel.parent()
                val = "/"+ sel.data()+ val
            print(val)


if __name__ == '__main__':
    import sys
    app = QtWidgets.QApplication(sys.argv)
    w = MainWindow()
    w.show()
    sys.exit(app.exec_())