Why does expanding a row call QAbstractItemModel's data DisplayRole twice in QTreeView

[Paul E]

If my tree is :
+A
++B
++C

When I expand row A, Qt calls data(DisplayRole) for its children twice.
1 0 C
0 0 B
0 0 A
0 0 B
1 0 C

Is there anyway to make Qt only check the children once?
1 0 C
0 0 B
0 0 A

Similarly, is there any way to prevent Qt from running data(DisplayRole) on the visible rows whenever the window loses/gains focus?

from PySide2 import QtGui, QtCore, QtUiTools, QtWidgets


class Node(object):
    def __init__(self, name="", parent=None):
        self._parent = parent
        self._name = name
        self._children = []

    def children(self):
        return self._children

    def hasChildren(self):
        return bool(self.children())

    def parent(self):
        return self._parent

    def name(self):
        return self._name

    def set_name(self, name):
        self._name = name

    def type_info(self):
        return 'NODE'

    def columnCount(self):
        return 1

    def child_count(self):
        return len(self._children)

    def add_child(self, child):
        self._children.append(child)
        child._parent = self

    def insert_child(self, position, child):
        if 0 <= position < child_count:
            self._children.insert(position, child)
            child._parent = self
            return True
        return False

    def remove_child(self, position):
        if 0 <= position < len(self._children):
            child = self._children.pop(position)
            child._parent = None
            return True
        return False

    def child(self, row):
        if 0 <= row < self.child_count():
            return self._children[row]

    def row(self):
        if self._parent is not None:
            return self._parent._children.index(self)
        return -1

    def find_child_by_name(self, name):
        for child in self._children:
            if child.name() == name:
                return child
        return None

    def log(self, tab_level=-1):
        output = ''
        tab_level += 1

        for i in range(tab_level):
            output += '\t'

        output += '|____' + self._name + '\n'

        for child in self._children:
            output += child.log(tab_level)

        tab_level -= 1

        return output

    def __repr__(self):
        return self.log()
class TemplateTreeModel(QtCore.QAbstractItemModel):
    def __init__(self):
        QtCore.QAbstractItemModel.__init__(self, None)
        self._root_node = Node()

    def headerData(self, section, orientation, role=QtCore.Qt.DisplayRole):
        if role == QtCore.Qt.DisplayRole:
            if section == 0:
                return 'Templates'
            else:
                return 'Type'

    def index(self, row, column, parent):
        if not self.hasIndex(row, column, parent):
            return QtCore.QModelIndex()
        node = parent.internalPointer() if parent.isValid() else self._root_node
        if node.children:
            return self.createIndex(row, column, node.child(row))
        else:
            return QtCore.QModelIndex()

    def parent(self, child):
        if not child.isValid():
            return QtCore.QModelIndex()
        node = child.internalPointer()
        if node.row() >= 0:
            return self.createIndex(node.row(), 0, node.parent())
        return QtCore.QModelIndex()

    def rowCount(self, parent=QtCore.QModelIndex()):
        node = parent.internalPointer() if parent.isValid() else self._root_node
        return node.child_count()

    def columnCount(self, parent=QtCore.QModelIndex()):
        return 1

    def hasChildren(self, parent= QtCore.QModelIndex()):
        node = parent.internalPointer() if parent.isValid() else self._root_node
        return node.hasChildren()

    def data(self, index, role=QtCore.Qt.DisplayRole):
        if index.isValid():
            if role == QtCore.Qt.DisplayRole:
                node = index.internalPointer()
                print index.row(), index.column(), node.name()
                return node.name()

    def setData(self, index, value, role=QtCore.Qt.EditRole):
        if role in (QtCore.Qt.EditRole,):
            node = index.internalPointer()
            node.set_name(value)
            self.dataChanged.emit(index, index)
            return True
        return False

    def indexFromItem(self, it):
        root_index = QtCore.QModelIndex()
        if isinstance(it, Node):
            parents = []
            while it is not self._root_node:
                parents.append(it)
                it = it.parent()
            root = self._root_node
            for parent in reversed(parents):
                root = root.find_child_by_name(parent.name())
                root_index =self.index(root.row(), 0, root_index)
        return root_index

    def item_from_path(self, path, sep):
        depth = path.split(sep)
        root = self._root_node
        for d in depth:
            root = root.find_child_by_name(d)
            if root is None: return None
        return root

    def appendRow(self, item, parent=None):
        self.appendRows([item], parent)

    def appendRows(self, items, parent=None):
        if isinstance(items, list):
            ix = self.indexFromItem(parent)
            self.insertRows(self.rowCount(ix), items, parent)

    def insertRows(self, position, items, parent=None):
        parent_index = self.indexFromItem(parent)
        self.beginInsertRows(parent_index, position, position + len(items) - 1)
        if parent is None:
            parent = self._root_node
        for item in items:
            parent.add_child(item)
        self.endInsertRows()

app = QtWidgets.QApplication.instance()
if not app:
    app = QtGui.QApplication(sys.argv)
w = QtWidgets.QTreeView()
model = TemplateTreeModel()
w.setModel(model)
model.appendRow(Node("A"))

structure_list = [['A', ['B', 'C']]]
for root, dirs in structure_list:
    depth = root.split(os.sep)
    if not root.startswith("A"):
        root = "A" + os.sep + root
    it = model.item_from_path(root, os.sep)
    model.appendRows([Node(_dir) for _dir in dirs], it)
w.show()

[VRonin]

For what I can see it gets called once for every children and once for every item that was moved as a result of the expansion.
It also gets called every time you hover over stuff.
The short answer is: data is needed for painting, it will be called A LOT, make it as efficient as possible.
Qt6 decreased the number of calls with multiData but they are still a lot