Why calling `print(self)` in constructor of a parent class is causing RuntimeError?

Question

Here's the traceback:

Traceback (most recent call last):
  File "/home/neon/pyqt6/myqt/src/app.py", line 13, in <module>
    window = AppWindow()
             ^^^^^^^^^^^
  File "/home/neon/pyqt6/myqt/src/app.py", line 7, in __init__
    super(AppWindow, self).__init__()
  File "/home/neon/pyqt6/myqt/src/ui_mainwindow.py", line 5, in __init__
    print(self)
RuntimeError: '__init__' method of object's base class (AppWindow) not called.

I'm trying to develop a personal QT6 based app in Python3.11. At some point I got stuck somewhere and decided to print(self) in one of the parents of a subclass, and that's what caused the above runtime error.

I've seen the duplicate. I have made attempts below to show why this runtime error should not happen in my opinion.

Here's how to reproduce the error:

1. Install PySide6 module in a virtual environment
2. Make file ui_mainwindow.py with following content:

from PySide6.QtWidgets import QLineEdit

class Ui_MainWindow(object):
    def __init__(self):
        # This call causes error
        print(self)
        
    # A sample widget. Irrelevant for error
    def makeui(self):
        self.line_edit = QLineEdit()

3. Make file app.py with following content:

from PySide6.QtWidgets import QMainWindow, QApplication
from ui_mainwindow import Ui_MainWindow
import sys

class AppWindow(QMainWindow, Ui_MainWindow):
    def __init__(self):
        super(AppWindow, self).__init__()
        self.makeui()
        self.setCentralWidget(self.line_edit)


if __name__ == "__main__":
    app = QApplication(sys.argv)
    window = AppWindow()
    window.show()
    app.exec()

4. Run app.py as python app.py and see the error reproduced.

Here's something unexpected. If I replace the call print(self) with anything else, let's say print(self.__class__.mro()) or print("Hello World") then no error occurs and print call works!

I'm unable to make sense of why print(self) is causing runtime error with that message.

---

I also tried to replicate the problem without qt6 and with only python based objects. Here's what I also tried and no error occurred even when print(self) was called in one parent's constructor.

1. Contents of main.py

from class_d import D

class A(object): 
    def __init__(self):
        super().__init__()

class B(A):
    def __init__(self):
        super().__init__()

class C(B):
    def __init__(self):
        super().__init__()

class E(C, D):
    def __init__(self):
        super(E, self).__init__()

e = E()

2. Contents of class_d.py

class D(object):
    def __init__(self):
        print(self)  # this call works as expected

Doing python main.py happily prints the repr of E object.

So why is that when qt objects are involved I am getting runtime error?