How to get the return value from a thread?

The function foo below returns a string 'foo'. How can I get the value 'foo' which is returned from the thread's target?

from threading import Threaddef foo(bar):    print('hello {}'.format(bar))    return 'foo'thread = Thread(target=foo, args=('world!',))thread.start()return_value = thread.join()

The "one obvious way to do it", shown above, doesn't work: thread.join() returned None.