QWidget::setLayout: Attempting to set QLayout "" on MainWindow "", which already has a layout

Question

I'm making an application in PyQt4 and this is my code so far:

import sys
from PyQt4 import QtGui, QtCore

class MainWindow(QtGui.QMainWindow):

    def __init__(self):
        super(MainWindow, self).__init__()
        self.initUi()

    def initUi(self):
        self.setWindowTitle('Main Menu')
        self.setFixedSize(1200, 625)
        self.firstWidgets()
        self.show()

    def firstWidgets(self):
        self.vbox1 = QtGui.QVBoxLayout()
        self.task1 = QtGui.QLabel('Check 1', self)
        self.task1CB = QtGui.QCheckBox(self)
        self.hbox1 = QtGui.QHBoxLayout()
        self.hbox1.addWidget(self.task1)
        self.hbox1.addWidget(self.task1CB)
        self.vbox1.addLayout(self.hbox1)

        self.setLayout(self.vbox1)


def main():
    application = QtGui.QApplication(sys.argv)
    gui = MainWindow()
    sys.exit(application.exec_())

if __name__=='__main__':
    main()

My problem is in MainWindow.firstWidgets(). I try to set a layout but I get an error even though that's my first time using .setLayout for that form, which confuses me.

QWidget::setLayout: Attempting to set QLayout "" on MainWindow "", which already has a layout

Answer 1

You can't set a QLayout directly on the QMainWindow. You need to create a QWidget and set it as the central widget on the QMainWindow and assign the QLayout to that.

wid = QtGui.QWidget(self)
self.setCentralWidget(wid)
layout = QtGui.QVBoxLayout()
wid.setLayout(layout)

NOTE: This is for Qt4 -- see the other answer on this question for the Qt5 updated code.

Answer 2

Just an update to Brenden Abel's answer:

QWidget and QVBoxLayout (for Python3, PyQt5) are now contained in the PyQt5.QtWidgets module and not the PyQt5.QtGui module.

So updated code:

wid = QtWidgets.QWidget(self)
self.setCentralWidget(wid)
layout = QtWidgets.QVBoxLayout()
wid.setLayout(layout)

Answer 3

This is an example using PyQt5

import sys
from PyQt5.QtCore import Qt
from PyQt5.QtWidgets import QApplication, QMainWindow, QVBoxLayout, QPushButton, QWidget


class MainWindow(QMainWindow):

    def __init__(self):
        super().__init__()
        self.setWindowTitle('My App')
        
        # Cannot set QxxLayout directly on the QMainWindow
        # Need to create a QWidget and set it as the central widget
        widget = QWidget()
        layout = QVBoxLayout()
        b1 = QPushButton('Red'   ); b1.setStyleSheet("background-color: red;")
        b2 = QPushButton('Blue'  ); b2.setStyleSheet("background-color: blue;")
        b3 = QPushButton('Yellow'); b3.setStyleSheet("background-color: yellow;")
        layout.addWidget(b1)
        layout.addWidget(b2)
        layout.addWidget(b3)
            
        widget.setLayout(layout)
        self.setCentralWidget(widget)


def main():
    app = QApplication(sys.argv)
    window = MainWindow()
    window.show()
    sys.exit(app.exec_())


if __name__ == '__main__':
    main()