future.result() freezes in add_done_callback()

Hi,

below is a simplified example of how calling future.result() in add_done_callback freezes.

c = Client()

def sleep():
    time.sleep(2)
    return 1

def cb(f):
    print(f)
    print('Result should be {}'.format(sleep()))
    print('Result was', f.result())

d = delayed(sleep)()
d.compute() # 1

c.compute(d).result() # 1

f2 = c.compute(d)
f2.add_done_callback(cb)
# <Future: status: finished, type: int, key: finalize-e24717a1ea49f34025d9f69a8d4b9e3e>
# Result should be 1

You never get the f.result() in cb. Execution freezes.

[mrocklin]

http://stackoverflow.com/questions/40477518/how-to-get-the-result-of-a-future-in-a-callback/40477862#40477862

[jlintusaari]

Yes, del f is what I wish to do in the callback, I was just confused by:

pitrou commented 3 hours ago del f would probably be safe, but would also not achieve anything particular.

So I wasn’t sure if del f would work in the callback since e.g. f.result() would not.