MRO error in PyQt5 when using multiple inheritance

Question

I'm new to PyQt5. I'm trying to create a single window with a menuBar (which inherits from the QMainWindow class) and some other widgets such as a button, some labels and text editors (which require the QWidget class). This is my code. When i try to add QMainWindow and QWidget as parent classes to may mainwindow class, i get an error saying: "TypeError: Cannot create a consistent method resolution order (MRO) for bases QWidget, QMainWindow". when i use only 1 parent class for mainwindow class it works fine, but in this case i have to use both QMainWindow and QWidget as parent classes.

import sys
from PyQt5.QtWidgets import QApplication, QWidget, QPushButton, QMessageBox, QAction, QMenu, QMainWindow, QTextEdit, QLineEdit, QGridLayout, QLabel

class mainwindow(QWidget, QMainWindow):
    def __init__(self):
        super().__init__()

        self.createUI()
    def createUI(self):
        mymenubar = self.menuBar()
        filemenu = mymenubar.addMenu("File")
        filemenu.addAction("help")
        filemenu.addAction("exit")



        contactlabel = QLabel("Contact:", self)
        contacttextedit = QLineEdit(self)
        countlabel = QLabel("Count:")
        counttextedit = QLineEdit()
        msglabel = QLabel("Your message here:")
        msgbox = QTextEdit()

        grid = QGridLayout()
        grid.setSpacing(10)




        grid.addWidget(contactlabel,1,0)
        grid.addWidget(contacttextedit,1,1)

        grid.addWidget(countlabel,2,0)
        grid.addWidget(counttextedit,2,1)

        grid.addWidget(msglabel,3,0)
        grid.addWidget(msgbox,3,1,5,1)

        self.setLayout(grid)
        self.setGeometry(300,300,300,300)
        self.setWindowTitle("Whatsapp Message Sender")
        self.show()



    # MODIFYING CLOSE EVENT SO IT ASKS BEFORE EXIT
    def closeEvent(self, event):
        reply = QMessageBox.question(self, "Message", "quit?", QMessageBox.Yes| QMessageBox.No, QMessageBox.Yes)

        if reply == QMessageBox.No:
            event.ignore()
        else:
            event.accept()



app = QApplication(sys.argv)
ex = mainwindow()
sys.exit(app.exec_())

What am i doing wrong? how can i use both QWidget and QMainwindow in my class?

eyllanesc eyllanesc · Accepted Answer · 2020-05-19T15:38:54

You have 2 errors:

QMainWindow already inherits from QWidget so multiple inheritance is unnecessary causing the error.
You should not establish a layout to a QMainWindow since it already has a predefined structure, instead you must create a central container where the layout is.

Considering the above, the solution is:

import sys
from PyQt5.QtWidgets import (
    QApplication,
    QWidget,
    QPushButton,
    QMessageBox,
    QAction,
    QMenu,
    QMainWindow,
    QTextEdit,
    QLineEdit,
    QGridLayout,
    QLabel,
)


class mainwindow(QMainWindow):
    def __init__(self):
        super().__init__()

        self.createUI()

    def createUI(self):
        mymenubar = self.menuBar()
        filemenu = mymenubar.addMenu("File")
        filemenu.addAction("help")
        filemenu.addAction("exit")

        contactlabel = QLabel("Contact:", self)
        contacttextedit = QLineEdit(self)
        countlabel = QLabel("Count:")
        counttextedit = QLineEdit()
        msglabel = QLabel("Your message here:")
        msgbox = QTextEdit()

        grid = QGridLayout()
        grid.setSpacing(10)

        grid.addWidget(contactlabel, 1, 0)
        grid.addWidget(contacttextedit, 1, 1)

        grid.addWidget(countlabel, 2, 0)
        grid.addWidget(counttextedit, 2, 1)

        grid.addWidget(msglabel, 3, 0)
        grid.addWidget(msgbox, 3, 1, 5, 1)

        # self.setLayout(grid)
        central_widget = QWidget()
        central_widget.setLayout(grid)
        self.setCentralWidget(central_widget)
        self.setGeometry(300, 300, 300, 300)
        self.setWindowTitle("Whatsapp Message Sender")
        self.show()

    # MODIFYING CLOSE EVENT SO IT ASKS BEFORE EXIT
    def closeEvent(self, event):
        reply = QMessageBox.question(
            self, "Message", "quit?", QMessageBox.Yes | QMessageBox.No, QMessageBox.Yes
        )

        if reply == QMessageBox.No:
            event.ignore()
        else:
            event.accept()


if __name__ == "__main__":

    app = QApplication(sys.argv)
    ex = mainwindow()
    sys.exit(app.exec_())

MRO error in PyQt5 when using multiple inheritance

2 Answers