How to set custom widget on QMainWindow in PyQt5

I am trying to have a custom widget displayed on pyQT main window but for some reason, I am getting two windows instead of the custom widget defined in the function years i.e. checkable combo box displayed in Dialog_01 (main window). Below is my code. Appreciate if someone has any direction or documentation I can look in.

import sys
from PyQt5 import QtGui, QtCore
from PyQt5.QtWidgets import QApplication, QMainWindow, QMenu, QVBoxLayout, QSizePolicy, QMessageBox, QWidget, QPushButton
from PyQt5 import QtWidgets
class CheckableComboBox(QtWidgets.QComboBox):
    closedPopup = QtCore.pyqtSignal()

    def __init__(self, parent=None):
        super(CheckableComboBox, self).__init__(parent)
        self.setView(QtWidgets.QListView(self))
        self.view().pressed.connect(self.handleItemPressed)
        self.setModel(QtGui.QStandardItemModel(self))

        firstItem = QtGui.QStandardItem("Select Years")
        firstItem.setBackground(QtGui.QBrush(QtGui.QColor(200, 200, 200)))
        firstItem.setSelectable(False)
        self.model().appendRow(firstItem)

    @QtCore.pyqtSlot(QtCore.QModelIndex)
    def handleItemPressed(self, index):
        item = self.model().itemFromIndex(index)
        if item.checkState() == QtCore.Qt.Checked:
            item.setCheckState(QtCore.Qt.Unchecked)
        else:
            item.setCheckState(QtCore.Qt.Checked)

    def checkedItems(self):
        l = []
        for i in range(self.model().rowCount()):
            it = self.model().item(i)
            if it.checkState() == QtCore.Qt.Checked:
                l.append(it.text())
        return l

    def hidePopup(self):
        self.closedPopup.emit()
        super(CheckableComboBox, self).hidePopup()
        QtCore.QTimer.singleShot(0, lambda: self.setCurrentIndex(0))
class Dialog_01(QMainWindow):
    def __init__(self):
        super().__init__()
        self.title = "Title"  
        self.top = 100        
        self.left = 100        
        self.width = 800      
        self.height = 500
        self.initUI()

    def initUI(self):
        self.setWindowTitle(self.title)
        self.setGeometry(self.top, self.left, self.width, self.height)
        self.years()
        self.location()
        self.MaxYears()

        self.layout = QtWidgets.QVBoxLayout(self)
        self.listWidget = self.comboBoxv
        self.layout.addWidget(self.listWidget)
        self.listWidget.show()
    def years(self):
        label0 = QLabel('Years', self)
        label0.move(50,50)
        label0.setGeometry(50,50,500,20)

        years = ["2017", "2018", "2019", "2020", "2021", "2022"]
        self.comboBoxv = CheckableComboBox()
        for i, area in enumerate(years):
            item = QtGui.QStandardItem(area)
            item.setCheckable(True)
            item.setCheckState(QtCore.Qt.Unchecked)
            self.comboBoxv.model().appendRow(item)        
        self.layout = QtWidgets.QVBoxLayout(self)
        self.setLayout(self.layout)
        self.listWidget = self.comboBoxv
        self.layout.addWidget(self.listWidget)
        self.listWidget.move(300, 50)


    @QtCore.pyqtSlot()
    def on_closedPopup(self):
        print(len(self.comboBoxv.checkedItems()))


    def location(self):  
        label1 = QLabel('Location', self)
        label1.move(50,150)
        label1.setGeometry(50,150,500,20)
        comboBox1 = QComboBox(self)
        for i in range(1,15):
            comboBox1.addItem("LOCATION %i" % i)
        comboBox1.move(300, 150)
        self.locationDrop=comboBox1
    def MaxYears(self):
        label2 = QLabel('Maximum Years', self)
        label2.move(50,250)
        label2.setGeometry(50,250,500,20)
        comboBox2 = QComboBox(self)
        for i in range(1, 10):
            comboBox2.addItem(str(i))
        comboBox2.move(300, 250)
        self.maxyearsDrop=comboBox2

if __name__ == '__main__':

    if not QApplication.instance():
        App = QApplication(sys.argv)
    else:
        App = QApplication.instance()
    ui = Dialog_01()
    ui.show()
    sys.exit(App.exec_())

Solution

You have to understand that programming is not placing nonsensical code, for example: what did you try to do with self.listWidget = self.comboBoxv?, as well as the above there are several inconsistencies.

On the other hand QMainWindow is a special widget since it already has a layout:

So you must create a centralwidget where you set the layout.

If you are going to use layouts you should no longer use the geometry, move or resize methods since the layout would handle those properties.

On the other hand I think you want to make a formula where there is a description and an editor, in that case Qt provides the QFormLayout.

import sys
from PyQt5 import QtCore, QtGui, QtWidgets

class CheckableComboBox(QtWidgets.QComboBox):
    closedPopup = QtCore.pyqtSignal()

    def __init__(self, parent=None):
        super(CheckableComboBox, self).__init__(parent)
        self.setView(QtWidgets.QListView(self))
        self.view().pressed.connect(self.handleItemPressed)
        self.setModel(QtGui.QStandardItemModel(self))

        firstItem = QtGui.QStandardItem("Select Years")
        firstItem.setBackground(QtGui.QBrush(QtGui.QColor(200, 200, 200)))
        firstItem.setSelectable(False)
        self.model().appendRow(firstItem)

    @QtCore.pyqtSlot(QtCore.QModelIndex)
    def handleItemPressed(self, index):
        item = self.model().itemFromIndex(index)
        if item.checkState() == QtCore.Qt.Checked:
            item.setCheckState(QtCore.Qt.Unchecked)
        else:
            item.setCheckState(QtCore.Qt.Checked)

    def checkedItems(self):
        l = []
        for i in range(self.model().rowCount()):
            it = self.model().item(i)
            if it.checkState() == QtCore.Qt.Checked:
                l.append(it.text())
        return l

    def hidePopup(self):
        self.closedPopup.emit()
        super(CheckableComboBox, self).hidePopup()
        QtCore.QTimer.singleShot(0, lambda: self.setCurrentIndex(0))

class Dialog_01(QtWidgets.QMainWindow):
    def __init__(self):
        super().__init__()
        self.title = "Title"  
        self.top, self.left, self.width, self.height = 100, 100, 800, 500
        self.initUI()

    def initUI(self):
        central_widget = QtWidgets.QWidget()
        self.flayout = QtWidgets.QFormLayout(central_widget)
        self.setCentralWidget(central_widget)

        self.setWindowTitle(self.title)
        self.setGeometry(self.top, self.left, self.width, self.height)
        self.years()
        self.location()
        self.MaxYears()

    def years(self):
        label = QtWidgets.QLabel('Years')

        years = ["2017", "2018", "2019", "2020", "2021", "2022"]
        self.comboBoxv = CheckableComboBox()
        for i, area in enumerate(years):
            item = QtGui.QStandardItem(area)
            item.setCheckable(True)
            item.setCheckState(QtCore.Qt.Unchecked)
            self.comboBoxv.model().appendRow(item)
        self.flayout.addRow(label, self.comboBoxv)

    @QtCore.pyqtSlot()
    def on_closedPopup(self):
        print(len(self.comboBoxv.checkedItems()))

    def location(self):  
        label = QtWidgets.QLabel('Location')
        combobox = QtWidgets.QComboBox()
        for i in range(1,15):
            combobox.addItem("LOCATION %i" % i)
        self.flayout.addRow(label, combobox)

    def MaxYears(self):
        label = QtWidgets.QLabel('Maximum Years')
        combobox = QtWidgets.QComboBox()
        for i in range(1, 10):
            combobox.addItem(str(i))
        self.flayout.addRow(label, combobox)

if __name__ == '__main__':
    App = QtWidgets.QApplication.instance()
    if App is None:
        App = QtWidgets.QApplication(sys.argv)
    ui = Dialog_01()
    ui.show()
    sys.exit(App.exec_())