How to properly submit data when using QDataWidgetMapper?

I'm using a custom model subclassed from QAbstractTableModel, my data is a list of dataclasses.

I've set up a simple GUI with a QListView and two QLineEdits, like so:

import sys
import dataclasses
from typing import List, Any
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *


@dataclasses.dataclass()
class StorageItem:

    field1: str = 'Item °1'
    field2: int = 42


class StorageModel(QAbstractTableModel):

    def __init__(self, parent=None):
        super().__init__(parent)

        self._data: List[StorageItem] = [StorageItem()]

    def data(self, index: QModelIndex, role: int = ...) -> Any:
        if not index.isValid():
            return

        item = self._data[index.row()]
        col = index.column()

        if role in {Qt.DisplayRole, Qt.EditRole}:
            if col == 0:
                return item.field1
            elif col == 1:
                return item.field2
            else:
                return None

    def setData(self, index: QModelIndex, value, role: int = ...) -> bool:
        print('dataChanged')

        if not index.isValid() or role != Qt.EditRole:
            return False

        item = self._data[index.row()]
        col = index.column()

        if col == 0:
            item.field1 = value
        elif col == 1:
            item.field2 = value

        self.dataChanged.emit(index, index)
        return True

    def flags(self, index: QModelIndex) -> Qt.ItemFlags:
        return Qt.ItemFlags(
            Qt.ItemIsEnabled | Qt.ItemIsSelectable | Qt.ItemIsEditable
        )

    def rowCount(self, parent=None) -> int:
        return len(self._data)

    def columnCount(self, parent=None) -> int:
        return len(dataclasses.fields(StorageItem))


class MainWindow(QMainWindow):

    def __init__(self, parent=None):
        super().__init__(parent)

        cent_widget = QWidget()
        self.setCentralWidget(cent_widget)

        # Vertical Layout
        v_layout = QVBoxLayout()
        v_layout.setContentsMargins(10, 10, 10, 10)

        self.model = StorageModel()

        # Listview
        self.listview = QListView()
        self.listview.setModel(self.model)
        v_layout.addWidget(self.listview)

        # Horizontal Layout
        h_layout = QHBoxLayout()
        h_layout.setContentsMargins(*[0]*4)

        self.field1 = QLineEdit()
        h_layout.addWidget(self.field1)

        self.field2 = QLineEdit()
        h_layout.addWidget(self.field2)

        v_layout.addLayout(h_layout)
        cent_widget.setLayout(v_layout)

        # Set Mapping
        mapper = QDataWidgetMapper()
        mapper.setSubmitPolicy(QDataWidgetMapper.AutoSubmit)
        mapper.setModel(self.model)
        mapper.addMapping(self.field1, 0)
        mapper.addMapping(self.field2, 1)
        mapper.toFirst()

        # self.field1.textChanged.connect(lambda: mapper.submit())


def main():
    app = QApplication(sys.argv)
    window = MainWindow()
    window.show()
    app.exec()


if __name__ == '__main__':
    main()

I'm trying to achieve that whenever I change the contents of the first QLineEdit, the list-view is updated as well.
From reading the documentation for QDataWidgetMapper I know that the model should be updated whenever the current widget loses focus, but it isn't. No matter what I enter in the edit-fields, the model's setData-method is never called.
Even if I edit the item in the list-view, the line-edit's contents don't change.

I discovered that when I connect the text-field's textChanged-signal to the mapper's submit-method, everything works, but the dataChanged-method is called three times, and I don't understand why.
Even stranger, now the text-field's contents are updated whenever I edit the item in the list-view, although connecting to the textChanged signal is (at least I think so) only a one-way connection.

What am I doing wrong? I'm clearly missing something, as QDataWidgetMapper's SubmitPolicy would be completely useless if this was the right way to do it.

Your mapper gets deleted as soon as __init__ returns, because there's no persistent reference for it. This is a common mistake for Qt objects in PyQt, usually caused by the fact that widgets added to a parent or layout become persistent even if there's no python reference, but the fact is that adding a widget to a layout actually creates a persistent reference (the parent widget takes "ownership", in Qt terms), thus preventing garbage collection.

Just make it an instance member or add the parent argument:

    self.mapper = QDataWidgetMapper()
    # alternatively (which is "safer" from the Qt point of view):
    mapper = QDataWidgetMapper(self)
    # but since you will probably need further access in any case:
    self.mapper = QDataWidgetMapper(self)