PyQt6: Open only one MDI Child Window at a time with subclassing

 This demo program shows how to create a custom event(close event for an MDI Child Window) by using the subclassing approach.

The output:

The code:

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import sys from PyQt6.QtCore import * from PyQt6.QtGui import * from PyQt6.QtWidgets import * sub_open = False class MainW(QMainWindow): count = 0 def __init__(self, parent=None): super(MainW, self).__init__(parent) self.mdi = QMdiArea() self.setCentralWidget(self.mdi) bar = self.menuBar() nsub = bar.addMenu("New") nsub.addAction("New") nsub.triggered[QAction].connect(self.waction) self.setWindowTitle("Post 40") #@f.pyqtSlot(str) def wclosed(self, text): global sub_open sub_open = False def waction(self, q): global sub_open if sub_open == True: return None sub_open = True if q.text() == "New": MainW.count = MainW.count + 1 sub = CustomSubWindow() # not QMdiSubWindow sub.setWidget(QTextEdit()) #sub.setAttribute(Qt.WA_DeleteOnClose) sub.setWindowTitle("subwindow" + str(MainW.count)) sub.subClosed.connect(self.wclosed) self.mdi.addSubWindow(sub) sub.show() class CustomSubWindow(QMdiSubWindow): subClosed = pyqtSignal(str) def closeEvent(self, event): self.subClosed.emit("") QMdiSubWindow.closeEvent(self, event) def main(): app = QApplication(sys.argv) ex = MainW() ex.show() sys.exit(app.exec()) if __name__ == '__main__': main()