PyQt crashes when QtItemDelegate subclass has two or more instances

Question

I'm implementing a delegate for a QTableView where two columns should be dropdowns where the user selects a value from an enum.

Below is an example:

from PyQt4 import QtGui, QtCore
import enum

Color = enum.Enum("Color", ("RED", "BLUE"))
Shape = enum.Enum("Shape", ("TRIANGLE", "CIRCLE"))


class EnumComboBoxDelegate(QtGui.QItemDelegate):
    def __init__(self, enum_cls, parent=None):
        super(EnumComboBoxDelegate, self).__init__(parent)
        self.enum_cls = enum_cls
        self.enum_objects = list(enum_cls)
        self.enum_names = [enum_obj.name for enum_obj in self.enum_objects]
        self.enum_values = [enum_obj.value for enum_obj in self.enum_objects]

    def createEditor(self, widget, option, index):
        editor = QtGui.QComboBox(widget)
        for user_friendly_name in self.enum_names:
            editor.addItem(user_friendly_name)
        return editor

    def setEditorData(self, editor, index):
        combobox_index, is_int = index.model().data(index, QtCore.Qt.EditRole).toInt()
        if is_int:
            editor.setCurrentIndex(combobox_index)
        else:
            editor.setCurrentIndex(0)

    def setModelData(self, editor, model, index):
        combobox_index = editor.currentIndex()
        if not combobox_index:
            combobox_index = 0

        model.setData(index, combobox_index, QtCore.Qt.EditRole)

    def updateEditorGeometry(self, editor, option, index):
        editor.setGeometry(option.rect)


class CentralWidget(QtGui.QWidget):
    def __init__(self, *args, **kwargs):
        super(CentralWidget, self).__init__(*args, **kwargs)

        main_layout = QtGui.QVBoxLayout()

        table_view = QtGui.QTableView()
        table_view.setSelectionBehavior(QtGui.QAbstractItemView.SelectRows)
        table_view.setSelectionMode(QtGui.QAbstractItemView.SingleSelection)

        table_model = QtGui.QStandardItemModel(2, 2, None)
        table_view.setModel(table_model)

        color_combo_delegate = EnumComboBoxDelegate(Color)
        table_view.setItemDelegateForColumn(0, color_combo_delegate)

        shape_combo_delegate = EnumComboBoxDelegate(Shape)
        table_view.setItemDelegateForColumn(1, shape_combo_delegate)

        main_layout.addWidget(table_view)

        self.setLayout(main_layout)


def run_self_contained_widget():
    import sys

    app = QtGui.QApplication(sys.argv)
    main_window = QtGui.QMainWindow()
    main_window.setCentralWidget(CentralWidget())
    main_window.show()
    sys.exit(app.exec_())


if __name__ == "__main__":
    run_self_contained_widget()

Now, if run, the code crashes on startup without traceback:

Process finished with exit code -1073741819 (0xC0000005)

If I change the delegates so they both use the same instance of EnumComboBoxDelegate, I get no errors:

color_combo_delegate = EnumComboBoxDelegate(Color)
table_view.setItemDelegateForColumn(0, color_combo_delegate)

shape_combo_delegate = EnumComboBoxDelegate(Shape)
table_view.setItemDelegateForColumn(1, color_combo_delegate)

What is causing this and how do I fix it? This code is using PyQt4 and Python 2.7, but it seems to be the same in Python 3.x

It's always good practice to ensure that delegates are kept alive. While a valid and common solution is to keep a reference as you suggested, a more appropriate (and Qt consistent) approach is to set the view as parent: you already have that argument in the subclass, just add it in the constructors. — musicamante, Aug 22 '23 at 06:14

score 0 · Answer 1 · answered Aug 22 '23 at 00:08

I can offer a partial solution, but I haven't figured out exactly why. It appears that color_combo_delegate gets garbage-collected at the end of CentralWidget.__init__(). If you add the following to EnumComboBoxDelegate:

    def __del__(self):
        print("Oh no! I am about to be destroyed! Enum: {}".format(self.enum_cls.__name__))

You get the output

Oh no! I am about to be destroyed! Enum: Color
Process finished with exit code -1073741819 (0xC0000005)

The fixed code is:

self.color_combo_delegate = EnumComboBoxDelegate(Color)
table_view.setItemDelegateForColumn(0, self.color_combo_delegate)

self.shape_combo_delegate = EnumComboBoxDelegate(Shape)
table_view.setItemDelegateForColumn(1, self.shape_combo_delegate)

It's unclear to me why a ref to shape_combo_delegate exists and not one for color_combo_delegate, but this appears to fix the issue.

PyQt crashes when QtItemDelegate subclass has two or more instances

1 Answers1