Example of closure issues when using instances without persistent references

pyqt-closure.py

from PyQt5.QtWidgets import *


class Dummy:
    def __init__(self):
        self.box = QGroupBox()
        self.label1 = QLabel()
        self.label2 = QLabel()

        layout = QVBoxLayout(self.box)
        layout.addWidget(self.label1)
        layout.addWidget(self.label2)

    def setText(self, text):
        self.label2.setText(text)


class Test(QWidget):
    def __init__(self):
        super().__init__()
        layout = QVBoxLayout(self)

        lineEdit = QLineEdit()
        layout.addWidget(lineEdit)

        dummy = Dummy()
        layout.addWidget(dummy.box)

        # NOTE: keep these commented for now, see below
        # dummy = BetterDummy()
        # layout.addWidget(dummy)

        # comment/uncomment either one of the following:
        # 1. this creates a closure, so "dummy" isn't deleted
        lineEdit.textChanged.connect(lambda t: dummy.label1.setText(t))
        # 2. this does *not* create a closure, as the only reference that is
        # actually used is the "setText" function
        # lineEdit.textChanged.connect(dummy.label1.setText)

        # point 2 is similar to the following:
        # setTextFunction = dummy.label1.setText
        # lineEdit.textChanged.connect(setTextFunction)

        # this will only work in case 1
        lineEdit.textChanged.connect(dummy.setText)
        # unless you change the two lines on top with those of the NOTE


class BetterDummy(QGroupBox):
    def __init__(self):
        super().__init__()
        self.label1 = QLabel()
        self.label2 = QLabel()

        layout = QVBoxLayout(self)
        layout.addWidget(self.label1)
        layout.addWidget(self.label2)

    def setText(self, text):
        self.label2.setText(text)        


app = QApplication([])

test = Test()
test.show()

app.exec()