The signal and slot within custom class including PyQt QWidget not working

Question

I have some trouble customizing a class including a QPushButton and QLabel, and I just want to set the QPushButton checkable and define a slot to its toggled signal, in addition, the custom class inherents QObject.

The code is shown below,

import sys
from PyQt5 import QtGui
from PyQt5.QtWidgets import *
from PyQt5.QtGui import *
from PyQt5.QtCore import pyqtSlot, QObject


class CustomButton(QPushButton):
    def __init__(self, label='', parent=None):
        super().__init__(label, parent)
        self.setCheckable(True)
        self.toggled.connect(self.update)

    def update(self, state):
        if state:
            self.setStyleSheet('background-color: green')
        else:
            self.setStyleSheet('background-color: red')

class ButtonCtrl(QObject):
    def __init__(self,  parent=None, label=''):
        super().__init__()
        if isinstance(parent, QLayout):
            col = QVBoxLayout()
            parent.addLayout(col)
        else:
            col = QVBoxLayout(parent)
        self.text = ['ON', 'OFF']
        self.label = QLabel(label)
        self.button = QPushButton('ON')
        # self.button = CustomButton('ON')
        col.addWidget(self.label)
        col.addWidget(self.button)
        self.button.setCheckable(True)
        self.button.setChecked(True)
        self.button.toggled.connect(self.update)
        self.update(True)
        self.label.setFont(QFont('Microsoft YaHei', 14))
        self.button.setFont(QFont('Microsoft YaHei', 12, True))
        self.button.toggle()

    # @pyqtSlot(bool)
    def update(self, state):
        if state:
            self.button.setText(self.text[0])
            self.button.setStyleSheet('background-color: green')
        else:
            self.button.setText(self.text[-1])
            self.button.setStyleSheet('background-color: red')


class Window(QWidget):
    def __init__(self, parent=None):
        super(Window, self).__init__(parent)
        # set the layout
        layout = QVBoxLayout(self)
        but = ButtonCtrl(layout, "Test")
        #self.but = ButtonCtrl(layout, "Test")
        btn = CustomButton()
        layout.addWidget(btn)

if __name__ == '__main__':
    app = QApplication(sys.argv)
    app.setStyle('Fusion')
    main = Window()
    main.show()
    sys.exit(app.exec_())

I have customized two buttons, named CustomButton(QPushButton) and ButtonCtrl(QObject), and I have tested the slot in the main window, however the background update slot works for CustomButton(QPushbutton) and does not work for ButtonCtrl(QObject), the slot function is not even invoked.

However, if I change the button member of ButtonCtrl(QObject) from QPushButton into my CustomButton(QPushButton), the it will work well in the main window. Furthermore, if the but in main window becomes a member of the main window class by setting self.but=ButtonCtrl(layout, "Test"), it will work as well.

I didn't find direct answer to it in Qt documentation which explains that

Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.

I am not sure if the lifetime of the but causing this effect, hope to get an answer, thank you.

user_6622587 · Answer

The problem is simple: The ButtonCtrl class object has a local scope so it will be destroyed, and why doesn't the same happen with the CustomButton class object? Well, because the ownership of a QWidget has its parent, and the parent of that button is the window, instead the ButtonCtrl object does not have it. In this case the solution is to extend the life cycle of the variable, and in the case of a QObject there are several options:

make the class member variable,
Place it in a container with a longer life cycle, or
establish a QObject parent with a longer life cycle.

Using the third alternative is just to change to:

class ButtonCtrl(QObject):
    def __init__(self,  parent=None, label=''):
        super().__init__(parent)
        # ...

The first option is the one commented on in your code:

self.but = ButtonCtrl(layout, "Test")

and the second is similar:

self.container = list()
but = ButtonCtrl(layout, "Test")
self.container.append(but)

The signal and slot within custom class including PyQt QWidget not working

1 Answers