Python, PySide, Qt implementation of saving/restoring window attributes (size, position, etc.) as settings (preferences) for an application. Mixin: make your custom subclass of QMainWindow multiply inherit this, and call these methods from appropriate places.

windowSettable.py

'''
Copyright 2013 Lloyd Konneker

License: LGPL
'''
from PySide.QtCore import QSettings


class WindowSettable(object):
  '''
  Mixin behavior for MainWindow: window attributes persist as Settings
  
  See Qt Application Example, where these are called readSettings() and writeSettings().
  
  Assert that QSettings have been established on app startup:
  QCoreApplication.setOrganizationName("Foo")
  QCoreApplication.setOrganizationDomain("foo.com")
  QCoreApplication.setApplicationName("Bar")
  '''
  
  
  def _readAndApplyWindowAttributeSettings(self):
    '''
    Read window attributes from settings,
    using current attributes as defaults (if settings not exist.)
    
    Called at QMainWindow initialization, before show().
    '''
    qsettings = QSettings()

    qsettings.beginGroup( "mainWindow" )

    # No need for toPoint, etc. : PySide converts types
    self.restoreGeometry(qsettings.value( "geometry", self.saveGeometry())) 
    self.restoreState(qsettings.value( "saveState", self.saveState()))
    self.move(qsettings.value( "pos", self.pos()))
    self.resize(qsettings.value( "size", self.size()))
    if qsettings.value( "maximized", self.isMaximized()) :
        self.showMaximized()

    qsettings.endGroup()
  
  

  def _writeWindowAttributeSettings(self):
    '''
    Save window attributes as settings.
    
    Called when window moved, resized, or closed.
    '''
    qsettings = QSettings()

    qsettings.beginGroup( "mainWindow" )
    qsettings.setValue( "geometry", self.saveGeometry() )
    qsettings.setValue( "saveState", self.saveState() )
    qsettings.setValue( "maximized", self.isMaximized() )
    if not self.isMaximized() == True :
        qsettings.setValue( "pos", self.pos() )
        qsettings.setValue( "size", self.size() )
    qsettings.endGroup()

Is this code snippet portable if qsettings.value( "maximized", self.isMaximized()):?

pyside/PySide#124 (comment)

note: qsettings.value( "maximized") read as a lowercase false string for me which registered as true for the if statement.
My way around this was:

def str2bool(self, strBool):
if type(strBool) == type(True):
return strBool
return strBool.lower() in ("yes", "true", "t", "1")

...

   maximized = qsettings.value("maximized", self.isMaximized())

    # apply settings
    self.restoreGeometry(qsettings.value("geometry", self.saveGeometry()))
    self.restoreState(qsettings.value("saveState", self.saveState()))
    self.move(qsettings.value( "pos", self.pos()))
    self.resize(qsettings.value( "size", self.size()))
    if self.str2bool(maximized) :
        self.showMaximized()

....

Here's my iteration, for the sake of knowledge and anyone who was having a bunch of trouble with maximizing.

    def _writeWindowAttributeSettings(self):
        '''
        Save window attributes as settings.

        Called when window moved, resized, or closed.
        '''
        self.settings.beginGroup("mainWindow")
        self.settings.setValue("pos", self.MainWindow.pos())
        self.settings.setValue("maximized", self.MainWindow.isMaximized())
        if not self.MainWindow.isMaximized():
            self.settings.setValue("size", self.MainWindow.size())

        self.settings.endGroup()

    def _readAndApplyWindowAttributeSettings(self):
        '''
        Read window attributes from settings,
        using current attributes as defaults (if settings not exist.)

        Called at QMainWindow initialization, before show().
        '''
        self.settings.beginGroup("mainWindow")
        # No need for toPoint, etc. : PySide converts types
        try:
            self.MainWindow.move(self.settings.value("pos"))
            if self.settings.value("maximized") in 'true':
                self.MainWindow.showMaximized()
            else:
                self.MainWindow.resize(self.settings.value("size"))
        except:
            pass
        self.settings.endGroup()

You'll notice I'm using self.MainWindow. change that as necessary for your code

It's worth mentioning that this all needs to happen after the widgets have been made. Obvious in hindsight; not so clear when trying to debug. Thanks @bootchk for putting all this info in one spot.

FWIW, here's where in the docs this stuff is mentioned: https://doc.qt.io/qtforpython-5/PySide2/QtCore/QSettings.html?highlight=qsettings#restoring-the-state-of-a-gui-application