python - Threading and Signals problem in PyQt -

i'm having problems communicating between threads in pyqt. i'm using signals communicate between 2 threads, sender , listener. sender sends messages, expected received listener. however, no messages receieved. can suggest might going wrong? i'm sure must simple, i've been looking around hours , not found anything. in advance!

from pyqt4 import qtcore,qtgui import time  class listener(qtcore.qthread):         def __init__(self):         super(listener,self).__init__()      def run(self):         # stay alive, waiting messages         print 'listener started'         while true:             print '...'             time.sleep(2)      def say_hello(self):         print ' --> receiver: hello world!'  class sender(qtcore.qthread):     # signal no arguments     signal = qtcore.pyqtsignal()      def __init__(self):         super(sender,self).__init__()         # create , start listener         self.listener = listener()         self.listener.start()         # connect signal         self.signal.connect(self.listener.say_hello)         # start thread         self.start()      def run(self):         print 'sender starting'         # send 5 signals         in range(5):             print 'sender -->'             self.signal.emit()             time.sleep(2)         # sender's work done         print 'sender finished' 

i'm not sure if need, works fine...

from pyqt4 import qtcore,qtgui import time  class listener(qtcore.qthread):     def __init__(self):         super(listener,self).__init__()      def run(self):         print('listener: started')         while true:             time.sleep(2)      def connect_slots(self, sender):         self.connect(sender, qtcore.signal('testsignal'), self.say_hello)      def say_hello(self):         print('listener: received signal')  class sender(qtcore.qthread):     def __init__(self):         super(sender,self).__init__()      def run(self):         in range(5):             print('sender: sending signal')             self.emit(qtcore.signal('testsignal'))             time.sleep(2)         print('sender: finished')  if __name__ == '__main__':     o_qapplication = qtgui.qapplication([])     my_listener = listener()     my_sender = sender()     my_listener.connect_slots(my_sender)     my_listener.start()     my_sender.start()     i_out = o_qapplication.exec_()