" RuntimeError: thread.__init__() not called" when subclassing threading.Thread

I need to run as many threads of class Observer as there are elements in list dirlist. When I run it python console it works all right.

class Observer(Thread):
    def run(self):
        naptime = random.randint(1,10)
        print(self.name + ' starting, running for %ss.' % naptime)
        time.sleep(naptime)
        print(self.name + ' done')

observers = {}
for d in dirlist:
    observers[d] = Observer()
    observers[d].start()

But when I try to do it from a Master thread which is supposed to spawn the Observer threads I get errors.

class Master(Thread):
    def __init__(self, dirlist):
        self.dirlist = dirlist
    def run(self):
        observers = {}
        for d in dirlist:
            observers[d] = Observer()
            observers[d].start()
        while True:
            time.sleep(3600)

master_thread = Master(dirlist)
master_thread.start()

The call to Master.start results in:

RuntimeError: thread.__init__() not called

This looks strange to me.
I am unable to understand whats the difference between both cases.
Can anybody figure out a solution to my problem ?

Somehow following doesn't produce an error, and I don't understand why.

class Master(Thread):
    def set(self, dirlist):
        self.dirlist = dirlist
    def run(self):
        observers = {}
        for d in dirlist:
            observers[d] = Observer()
            observers[d].start()
        while True:
            time.sleep(3600)

master_thread = Master()
master_thread.set(dirlist)
master_thread.start()