PySide: QSettings as class variable has inconsistent behavior - pyside

Question

We write many PySide scripts for Maya 2015, and we save the settings using QSettings. Normally we create the QSettings object in our "readSettings" and "writeSettings" functions. Today I tried making the QSettings object a class variable. But that caused some strange effects. Certain values that normally come back as <type 'unicode'> started coming back as <type 'bool'>, but not all the time!
Here is a test script I wrote to illustrate the problem:
import shiboken
from PySide import QtGui, QtCore
from maya import OpenMayaUI
#------------------------------------------------------------------------------
def getMayaMainWindow():
parentWindow = OpenMayaUI.MQtUtil.mainWindow()
if parentWindow:
return shiboken.wrapInstance(long(parentWindow), QtGui.QWidget)
#------------------------------------------------------------------------------
class TestQSettingsWin(QtGui.QMainWindow):
def __init__(self, parent=getMayaMainWindow()):
super(TestQSettingsWin, self).__init__(parent)
self.setWindowTitle('Test QSettings')
self.setObjectName('testAllMMessagesWindow')
self.centralWidget = QtGui.QWidget()
self.setCentralWidget(self.centralWidget)
self.mainLayout = QtGui.QVBoxLayout(self.centralWidget)
self.checkBox = QtGui.QCheckBox('check box')
self.mainLayout.addWidget(self.checkBox)
self.readSettings()
def closeEvent(self, event):
self.writeSettings()
def getQSettingsLocation(self):
raise NotImplementedError('Subclasses of TestQSettingsWin need to '
'implement "getQSettingsLocation"".')
def readSettings(self):
setting = self.getQSettingsLocation()
self.restoreGeometry(setting.value('geometry'))
self.restoreState(setting.value('windowState'))
print type(setting.value('checkBox'))
def writeSettings(self):
setting = self.getQSettingsLocation()
setting.setValue('geometry', self.saveGeometry())
setting.setValue('windowState', self.saveState())
setting.setValue('checkBox', self.checkBox.isChecked())
#------------------------------------------------------------------------------
class TestQSettingsClassVar(TestQSettingsWin):
savedSettings = QtCore.QSettings(QtCore.QSettings.IniFormat,
QtCore.QSettings.UserScope,
"Test",
"TestQSettings1")
def getQSettingsLocation(self):
return self.savedSettings
#------------------------------------------------------------------------------
class TestQSettingsDefScope(TestQSettingsWin):
def getQSettingsLocation(self):
setting = QtCore.QSettings(QtCore.QSettings.IniFormat,
QtCore.QSettings.UserScope,
"Test",
"TestQSettings3")
return setting
#------------------------------------------------------------------------------
def showTestWindows():
test1 = TestQSettingsClassVar()
test1.setAttribute(QtCore.Qt.WA_DeleteOnClose, True)
test1.show()
test2 = TestQSettingsDefScope()
test2.setAttribute(QtCore.Qt.WA_DeleteOnClose, True)
test2.show()
And here are the results from running it in an interactive session:
>>> import testQSettings
>>> testQSettings.showTestWindows()
<type 'NoneType'>
<type 'NoneType'>
>>> testQSettings.showTestWindows()
<type 'bool'>
<type 'unicode'>
>>> testQSettings.showTestWindows()
<type 'bool'>
<type 'unicode'>
>>> reload(testQSettings)
# Result: <module 'testQSettings' from 'C:/Users/becca/Documents/maya/2015-x64/scripts\testQSettings.pyc'> #
>>> testQSettings.showTestWindows()
<type 'unicode'>
<type 'unicode'>
>>> testQSettings.showTestWindows()
<type 'bool'>
<type 'unicode'>
>>> testQSettings.showTestWindows()
<type 'bool'>
<type 'unicode'>
As you can see, creating the QSettings object whenever it is needed consistently returns a <type 'unicode'> result for the data value. But creating the QSettings object as a class variable returns a <type 'bool'> result except when the module is reloaded, and then it returns a <type 'unicode'>.
Can anyone explain this strange behavior? Is there a rule that I should not make the QSettings object a class variable?

Answer 1

The settings object has to serialize the various different types of value to bytes before writing it to disk. This is usually done when the settings object is deleted (or, if an event loop is running, unsaved data may be periodically flushed to disk).
All the time that the unsaved data has not been flushed to disk, whenever you call settings.value(), it will return the unserialized value in its original type.
It may be possible to forcibly flush the data yourself by calling settings.sync(), but I would strongly advise against trying this. You should always create a new QSettings object whenever you want to read or write values, and ensure that it gets deleted after you've used it. That should be enough to guarantee consistency.