如何实现类方法的多次覆盖

Question

In my exampe below, I want to set the alignmet for the first and the second column of the tableview in two different classes. But it does not work as I want. In my example is only the alignment of the first column set.

The reason why I want to do this is, because I want to make some base settings in a base-widget-class and then, if needed, I want to make addition settings in a subclassed widget.

import sys
from PyQt5.QtGui import QStandardItemModel
from PyQt5.QtCore import Qt
from PyQt5.QtWidgets import QTableView, QApplication, QWidget, QVBoxLayout


class MyWidget(QWidget):

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

        model = FirstModel(4, 2)
        model.setData(model.index(0, 0), "a", Qt.EditRole)
        model.setData(model.index(0, 1), 2, Qt.EditRole)
        model.setData(model.index(1, 0), "b", Qt.EditRole)
        model.setData(model.index(1, 1), 4, Qt.EditRole)
        model.setData(model.index(2, 0), "c", Qt.EditRole)
        model.setData(model.index(2, 1), 6, Qt.EditRole)
        model.setData(model.index(3, 0), "d", Qt.EditRole)
        model.setData(model.index(3, 1), 8, Qt.EditRole)

        tableview = QTableView()
        tableview.setModel(model)

        layout = QVBoxLayout()
        layout.addWidget(tableview)
        self.setLayout(layout)


class SecondModel(QStandardItemModel):

    def data(self, index, role=Qt.DisplayRole):
        super().data(index, role)

        # set Alignment for column 1
        if role == Qt.TextAlignmentRole:
            if index.column() == 1:
                return Qt.AlignHCenter | Qt.AlignVCenter

        return QStandardItemModel.data(self, index, role)


class FirstModel(SecondModel):

    def data(self, index, role=Qt.DisplayRole):
        super().data(index, role)

        # set Alignment for column 0
        if role == Qt.TextAlignmentRole:
            if index.column() == 0:
                return Qt.AlignHCenter | Qt.AlignVCenter

        return QStandardItemModel.data(self, index, role)


if __name__ == '__main__':

    app = QApplication(sys.argv)
    table_view = MyWidget()
    table_view.show()

    sys.exit(app.exec_())

Answer 1

In this answer I will start explaining the solution and thereby understand the OP error.

Solution:

Before override a method, it must be understood what that method does. The data() method aims to return information related to a role and a QModelIndex.

Considering the above in SecondModel, you must return Qt.AlignHCenter | Qt.AlignVCenter Qt.AlignHCenter | Qt.AlignVCenter when the role is Qt.TextAlignmentRole and the column is "1", in the other cases you want to behave like the parent(QStandardItemModel) then you must use super() :

class SecondModel(QStandardItemModel):
    def data(self, index, role=Qt.DisplayRole):
        if role == Qt.TextAlignmentRole:
            if index.column() == 1:
                return Qt.AlignHCenter | Qt.AlignVCenter
        return super().data(index, role)

The same should be done with FirstModel:

class FirstModel(SecondModel):
    def data(self, index, role=Qt.DisplayRole):
        if role == Qt.TextAlignmentRole:
            if index.column() == 0:
                return Qt.AlignHCenter | Qt.AlignVCenter
        return super().data(index, role)

---

OP error explanation:

class SecondModel(QStandardItemModel):
    def data(self, index, role=Qt.DisplayRole):
        super().data(index, role)
        # set Alignment for column 1
        if role == Qt.TextAlignmentRole:
            if index.column() == 1:
                return Qt.AlignHCenter | Qt.AlignVCenter
        return QStandardItemModel.data(self, index, role)

In this code it is equivalent to mine in functionality but it has an unnecessary code since super().data(index, role) only returns something but the OP does not use the result. Also in this case super().data(index, role) is the same as QStandardItemModel.data(self, index, role) .

class FirstModel(SecondModel):
    def data(self, index, role=Qt.DisplayRole):
        super().data(index, role)
        # set Alignment for column 0
        if role == Qt.TextAlignmentRole:
            if index.column() == 0:
                return Qt.AlignHCenter | Qt.AlignVCenter
        return QStandardItemModel.data(self, index, role)

In this case super().data(index, role) does not equal QStandardItemModel.data(self, index, role) because the parent of FirstModel is not QStandardItemModel but SecondModel . So by not calling the method of SecondModel you are deleting the inherited behavior of SecondModel .