window freezes after clicking of button in python gtk3

1import threading
2
3def on_next2_clicked(self,button):
4    def my_thread(obj):
5        cmd = "My Command"
6        proc = subprocess.Popen(cmd,shell=True, stdout=subprocess.PIPE)
7        while True:
8                line = proc.stdout.read(2)
9                if not line:
10                    break
11                obj.fper = float(line)/100.0
12                obj.ui.progressbar1.set_fraction(obj.fper)
13        print "Done"
14
15    threading.Thread(target=my_thread, args=(self,)).start()