Проверьте ваш документ на PyQt4/doc/html/qt.html#MouseButton-enum
Constant Value
Qt.NoButton 0x00000000
Qt.LeftButton 0x00000001
Qt.RightButton 0x00000002
Qt.MidButton 0x00000004
Qt.MiddleButton MidButton
Qt.XButton1 0x00000008
Qt.XButton2 0x00000010
Простой пример:
>>> from PyQt4 import QtCore, QtGui
>>> app=QtGui.QApplication([])
>>> mouse_state=app.mouseButtons()
>>> mouse_state==QtCore.Qt.NoButton
True
>>> int(mouse_state)
0