Showing a test, inside and outside of Maya, of child windows still remaining alive after being closed. Test can toggle the WA_DeleteOnClose attribute on each child widget:

child_window_test.py

#!/usr/bin/env python

"""
Showing a test, inside and outside of Maya,
of child windows still remaining alive after being closed. 

Test can toggle the WA_DeleteOnClose attribute on each child widget:

# Command line (dont delete)
$ ./child_window_test.py

# Command line (delete)
$ ./child_window_test.py -delete

# Inside Maya
import child_window_test
child_window_test.start(delete=True)

"""
from PySide import QtCore 
from PySide import QtGui 

IS_MAYA = QtGui.qApp is not None

if IS_MAYA:
    import shiboken
    import maya.OpenMayaUI as mui
 

def getMayaWindow():
    ptr = mui.MQtUtil.mainWindow()
    return shiboken.wrapInstance(long(ptr), QtGui.QWidget)
 

def checkChildren(parent, timer):
    count = len([c for c in parent.children() if isinstance(c, TestWindow)])
    print "Num. of TestWindows:", count

    if not count:
        timer.stop()


class TestWindow(QtGui.QMainWindow):
   
    def __init__ (self, parent=None) :
        QtGui.QMainWindow.__init__(self, parent=parent)
        self.setObjectName("testWindow")
        self.layout = QtGui.QHBoxLayout()
        self.label = QtGui.QLabel("test panel")
        self.layout.addWidget(self.label)
        self.setLayout(self.layout)


def start(delete=False):

    # Create a main window if we are running outside Maya
    if IS_MAYA:
        mainWindow = getMayaWindow()
    else:
        app = QtGui.QApplication([])
        mainWindow = QtGui.QMainWindow()
        mainWindow.setWindowTitle("I am the Maya Main Window")
        mainWindow.resize(600,400)
        mainWindow.show()
        mainWindow.raise_()

    # Make some child windows
    for i in xrange(5):
        window = TestWindow(parent=mainWindow)
        window.setAttribute(QtCore.Qt.WA_DeleteOnClose, delete)
        window.setWindowTitle("I am a test window")
        window.resize(300,200)
        window.show()
        window.raise_()

    # Check the children every 1.5 seconds
    timer = QtCore.QTimer(mainWindow)
    timer.timeout.connect(lambda: checkChildren(mainWindow, timer))
    timer.start(1500)

    # Stop the check timer after 15 seconds
    QtCore.QTimer.singleShot(15 * 1000, timer.stop)

    if not IS_MAYA:
        app.exec_()


if __name__ == "__main__":
    import sys

    args = sys.argv[1:]
    delete = '-delete' in args
    start(delete)