calling a parent method from a child widget in pyside/pyqt

Question

I'm trying to call a method of a parent class from within a child class. Specifically, my parent class is a PySide.QtGui.QMainWindow object, and my child class is a PySide.QtGui.QWidget object; the latter is set to be the central widget of the former. I'm trying to connect a button within the child to a method in the parent class. This has worked for me in the past using self.parent().method_name, but it doesn't work in the example below and I don't understand why:

import sys
from PySide import QtGui, QtCore


class MainWindow(QtGui.QMainWindow):

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

        self.do_something() #sanity check
        self.cw = ChildWidget()
        self.setCentralWidget(self.cw)
        self.show()

    def do_something(self):

        print 'doing something!'


class ChildWidget(QtGui.QWidget):

    def __init__(self):
        super(ChildWidget, self).__init__()

        self.button1 = QtGui.QPushButton()
        self.button1.clicked.connect(self.do_something_else)

        self.button2 = QtGui.QPushButton()
        self.button2.clicked.connect(self.parent().do_something)

        self.layout = QtGui.QVBoxLayout()
        self.layout.addWidget(self.button1)
        self.layout.addWidget(self.button2)
        self.setLayout(self.layout)
        self.show()

    def do_something_else(self):

        print 'doing something else!'


def main():
    app = QtGui.QApplication(sys.argv)
    ex = MainWindow()
    sys.exit(app.exec_())

if __name__ == '__main__':
    main()

Here is the error:

self.button2.clicked.connect(self.parent().do_something)
AttributeError: 'NoneType' object has no attribute 'do_something'

score 8 · Accepted Answer · answered Jun 19 '14 at 17:16

You never set your MainWindow as the parent of your ChildWidget. So self.parent() evaluates to None and therefore has no function do_something.

Try:

import sys
from PySide import QtGui, QtCore


class MainWindow(QtGui.QMainWindow):

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

        self.do_something() #sanity check
        self.cw = ChildWidget(self)
        self.setCentralWidget(self.cw)
        self.show()

    def do_something(self):

        print 'doing something!'


class ChildWidget(QtGui.QWidget):

    def __init__(self, parent):
        super(ChildWidget, self).__init__(parent)

        self.button1 = QtGui.QPushButton()
        self.button1.clicked.connect(self.do_something_else)

        self.button2 = QtGui.QPushButton()
        self.button2.clicked.connect(self.parent().do_something)

        self.layout = QtGui.QVBoxLayout()
        self.layout.addWidget(self.button1)
        self.layout.addWidget(self.button2)
        self.setLayout(self.layout)
        self.show()

    def do_something_else(self):

        print 'doing something else!'


def main():
    app = QtGui.QApplication(sys.argv)
    ex = MainWindow()
    sys.exit(app.exec_())

if __name__ == '__main__':
    main()

I don't undersand in `def __init__(self, parent):` and `super(ChildWidget, self).__init__(parent)` what _parent_ is? Where is it defined? Is it implicit `MainWindow` from `self.cw = ChildWidget(self)`? — Hrvoje T, Aug 16 '15 at 14:07
That is how you set the parent. The instance of `ChildWidget` is initialised with the parent `self` (i.e., the instance of `MainWindow`). At first I was confused between the python concept of base classes and the Qt concept of parents and children. — sammosummo, Aug 16 '18 at 19:02

score 5 · Answer 2 · answered Jun 19 '14 at 17:12

Here the working code:

import sys
from PySide import QtGui, QtCore


class MainWindow(QtGui.QMainWindow):

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

        self.do_something() #sanity check
        self.cw = ChildWidget(self)
        self.setCentralWidget(self.cw)
        self.show()

    def do_something(self):

        print 'doing something!'


class ChildWidget(QtGui.QWidget):

    def __init__(self, parent=None):
        super(ChildWidget, self).__init__(parent=parent)

        self.button1 = QtGui.QPushButton()
        self.button1.clicked.connect(self.do_something_else)

        self.button2 = QtGui.QPushButton()
        self.button2.clicked.connect(self.parent().do_something)

        self.layout = QtGui.QVBoxLayout()
        self.layout.addWidget(self.button1)
        self.layout.addWidget(self.button2)
        self.setLayout(self.layout)
        self.show()

    def do_something_else(self):

        print 'doing something else!'


def main():
    app = QtGui.QApplication(sys.argv)
    ex = MainWindow()
    sys.exit(app.exec_())

if __name__ == '__main__':
    main()

Hopes, can help you.

calling a parent method from a child widget in pyside/pyqt

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