QWidget cannot display on QMainWindow instance PyQt5 - python-3.x

Question

I am learning PyQt5 now and tried to do something little on my own. I have made a very basic custom toolbox, which has just 6 QPushButtons buttons on it, which inherits from QWidget class.
My problem is that I can't display my toolbox on my QMainWidow instance. Let me show you what I did;
import sys
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *
from PyQt5.QtGui import *
class ToolBox(QWidget):
def __init__(self):
super().__init__()
self.initUI()
def initUI(self):
btn = [QPushButton('B', self) for i in range(6)]
for Btn in btn:
Btn.resize(30, 30)
self.resize(60, 90)
k = 0
for i in range(6):
btn[i].move((i%2)*30, k*30)
k += 1 if i % 2 == 1 else 0
self.show()
class MainWindow(QMainWindow):
def __init__(self):
super().__init__()
self.initUI()
def initUI(self):
self.resize(300, 200)
self.statusBar().showMessage('Ready!')
exitAction = QAction(QIcon('idea.png'), 'Exit', self)
exitAction.setStatusTip('Exit application')
exitAction.setShortcut('Ctrl+Q')
exitAction.triggered.connect(qApp.quit)
menuBar = self.menuBar()
fileMenu = menuBar.addMenu('File')
fileMenu.addAction(exitAction)
t = ToolBox()
t.move(150, 150)
t.show() #With and without this line, it doesn't work.
self.show()
if __name__ == '__main__':
app = QApplication(sys.argv)
m = MainWindow()
sys.exit(app.exec_())

Answer 1

You just need to position your widget somewhere in the QMainWindow canvas. All you have to do is position it in the MainWindow. Just for an example, I use setCentralWidget() to position your QWidget.
import sys
from PyQt5.QtWidgets import *
from PyQt5.QtCore import *
from PyQt5.QtGui import *
class ToolBox(QWidget):
def __init__(self):
super().__init__()
self.initUI()
def initUI(self):
btn = [QPushButton('B', self) for i in range(6)]
for Btn in btn:
Btn.resize(30, 30)
self.resize(60, 90)
k = 0
for i in range(6):
btn[i].move((i%2)*30, k*30)
k += 1 if i % 2 == 1 else 0
class MainWindow(QMainWindow):
def __init__(self):
super().__init__()
self.initUI()
def initUI(self):
self.resize(300, 200)
self.statusBar().showMessage('Ready!')
exitAction = QAction(QIcon('idea.png'), 'Exit', self)
exitAction.setStatusTip('Exit application')
exitAction.setShortcut('Ctrl+Q')
exitAction.triggered.connect(qApp.quit)
menuBar = self.menuBar()
fileMenu = menuBar.addMenu('File')
fileMenu.addAction(exitAction)
t = ToolBox()
self.setCentralWidget(t)
if __name__ == '__main__':
app = QApplication(sys.argv)
m = MainWindow()
m.show()
sys.exit(app.exec_())