Does the main window only update after a slot subroutine returns?

Hope this helps I finally figured it out. You have to select the main window and hit F4 to add the function to the slots for the window.

With these it helps Try to use QWidget::setGeometry.

it should still fix some issue To cease execution of your macro immediately, without returning to the calling procedure, you can use the End statement.

With these it helps I think the while loop interferes the Tornado IO_loop. I advice you to use add_periodic_callback instead (Bokeh v1.1.0)

from bokeh.models import Column
from bokeh.plotting import curdoc
from bokeh.models.widgets import Button, Toggle, CheckboxGroup
import time

# def start_loop():
#     while (not button3.active) and (len(cb.active)):
#         time.sleep(1)
#         print(button3.active)
#         print(cb.active)

def check_status():
    print(button3.active)
    print(cb.active)

# button1 = Button(label = "start")
# button1.on_click(start_loop)

button2 = Button(label = "check status")
button2.on_click(check_status)

button3 = Toggle(label = "stop")
cb = CheckboxGroup(labels = ['stop'], active = [0])

curdoc().add_root(Column(button2, button3, cb))
curdoc().add_periodic_callback(check_status, 1000)

I wish this help you Explanation:
When you have created the .ui you have only pointed out that there is a connection between the clicked signal and the init method of the widget, but when you load it using loadUi() and do not pass it as a second parameter a QWidget will use the base class of the design, in your case QWidget, which clearly has no "init" method throwing you that error.

# -*- coding: utf-8 -*-
import os
import sys

from PyQt5 import QtCore, QtWidgets, uic as pyuic


class Widget(QtWidgets.QWidget):
    def __init__(self, parent=None):
        super().__init__(parent)
        current_dir = os.path.dirname(os.path.realpath(__file__))
        uifile = os.path.join(current_dir, "serialTandemGUI.ui")
        pyuic.loadUi(uifile, self)

    @QtCore.pyqtSlot()
    def init(self):
        print("init")


class MainWindow(QtWidgets.QMainWindow):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.form_widget = Widget()
        _widget = QtWidgets.QWidget()
        _layout = QtWidgets.QVBoxLayout(_widget)
        _layout.addWidget(self.form_widget)
        self.setCentralWidget(_widget)


if __name__ == "__main__":
    app = QtWidgets.QApplication.instance() or QtWidgets.QApplication(sys.argv)
    app.aboutToQuit.connect(app.deleteLater)

    w = MainWindow()
    w.show()

    sys.exit(app.exec_())