https://stackoverflow.com/questions/46354341/attribute-error-bool-object-has-no-attribute-signal

main.py

import sys
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *
from PyQt5.QtGui import *
import cv2

class ShowVid(QObject):
    signal = pyqtSignal(QImage)        

    def __init__(self):
        super().__init__()

    @pyqtSlot()
    def startVideo(self):
        camera_port = 0
        camera = cv2.VideoCapture(camera_port)          
        run_video = True
        while run_video:
            ret, image = camera.read()
            if ret:
                color_swapped_image = cv2.cvtColor(image, cv2.COLOR_BGR2RGB)

                height, width, _ = color_swapped_image.shape
                qt_image = QImage(color_swapped_image.data,
                                  width,
                                  height,
                                  color_swapped_image.strides[0],
                                  QImage.Format_RGB888)
                self.signal.emit(qt_image)

    def stop(self):
        self.run_video = True
        self.quit()
        self.wait()

class VidViewer(QWidget):
    def __init__(self):
        super().__init__()
        self.layout = QGridLayout()
        self.image = QImage()        
        self.initUI()

    def paintEvent(self, _):
        print("paint image")
        if not self.image.isNull():
            painter = QPainter(self)
            painter.drawImage(0, 0, self.image)
        # self.paintImage = QImage()

    def initUI(self):
        print("INIT UI")  
        #image = self.image
        self.push_button = QPushButton("Start")
        self.layout.addWidget(self.push_button, 0, 0,1,1) 

        self.setLayout(self.layout) 
        self.setWindowTitle('Test')
        self.show()

    @pyqtSlot(QImage)
    def setImage(self, image):
        self.image = image
        self.update()
        QApplication.processEvents()


class Main(QMainWindow):
    def __init__(self):
        super().__init__()
        self.central_widget = QStackedWidget()
        self.setCentralWidget(self.central_widget)

        self.viewer = VidViewer()
        self.vid = ShowVid()
        self.viewer.push_button.clicked.connect(self.vid.startVideo)

        self.central_widget.addWidget(self.viewer)       
        self.central_widget.setCurrentWidget(self.viewer) 

        self.vid.signal.connect(self.viewer.setImage)  

        self.setGeometry(10,10,1280,720)        
        self.setFixedSize(1280,720) 

    def closeEvent(self, event):
        self.vid.stop()
        QMainWindow.closeEvent(self, event)

app = QApplication(sys.argv)
run = Main()   
run.show()

sys.exit(app.exec_())