try:
    import PySide6.QtCore as QtCore
    import PySide6.QtWidgets as QtWidgets
except ImportError:
    import PySide2.QtCore as QtCore
    import PySide2.QtWidgets as QtWidgets
​
class MyTextItem(QtWidgets.QGraphicsTextItem):
    def __init__(self, text, parent=None):
        super(MyTextItem, self).__init__(parent)
        self.isPlain = True
        self.itemText = text
        self.setPlainText(text)
​
    def toggleMarkdown(self):
        if not self.isPlain:
            self.setPlainText(self.itemText)
        else:
            d = self.document()
            d.setMarkdown(self.itemText)
        self.isPlain = not self.isPlain
​
class Main(QtWidgets.QWidget):
    def __init__(self, parent=None):
        super(Main, self).__init__()
        w = QtWidgets.QGraphicsView()
        s = QtWidgets.QGraphicsScene()
        myItem = MyTextItem('This is a paragraph.\n\nThis is another paragraph.')
        myItem.setPos(0, 30)
        s.addItem(myItem)
        w.setScene(s)
        self.toggleButton = QtWidgets.QPushButton("Toggle view mode")
        self.toggleButton.clicked.connect(myItem.toggleMarkdown)
        self.layout = QtWidgets.QVBoxLayout()
        self.layout.addWidget(self.toggleButton)
        self.layout.addWidget(w)
        self.setLayout(self.layout)
        self.resize(300, 200)
​
if __name__ == '__main__':
    app = QtWidgets.QApplication()
    win = Main()
    win.show()
    app.exec_()