Internal variable __started remains set to True/1 even
if thread is already terminated, this situation will
generate assertion error when .start is invoked second
time.

demo.py

import time
import threading

done = threading.Event()
def demo():
	print 'bip'
	done.set()

t=threading.Thread(target=demo)
t.start()
while not done.isSet():
	time.sleep(0.1)
done.clear()
print 'returned'
t.start()