Pass the QMainWindow the WindowStaysOnTopHint
window flag (or use setWindowFlags).
As in the name, this is a hint to the windowing manager (not a hard guarantee).
Simplest possible example:
import sys
from PyQt4 import QtGui, QtCore
class mymainwindow(QtGui.QMainWindow):
def __init__(self):
QtGui.QMainWindow.__init__(self, None, QtCore.Qt.WindowStaysOnTopHint)
app = QtGui.QApplication(sys.argv)
mywindow = mymainwindow()
mywindow.show()
app.exec_()