Passing data across callbacks in ThreadPoolExecutor

Ian Kelly ian.g.kelly at
Wed Feb 17 11:24:31 EST 2016

On Tue, Feb 16, 2016 at 2:48 PM, Joseph L. Casale
<jcasale at> wrote:
> What is the pattern for chaining execution of tasks with ThreadPoolExecutor?
> Callbacks is not an adequate facility as each task I have will generate new output.

Can you specify in more detail what your use case is?

If you don't mind having threads sitting around waiting, you can just
submit each chained task at the start and have each task wait on the
futures of its dependencies. The advantage of this is that it's easy
to conceptualize the dependency graph of the tasks. The disadvantage
is that it eats up extra threads. You'll probably want to increase the
size of the thread pool to handle the waiting tasks (or use a separate
ThreadPoolExecutor for each chained task).


def task2(input, f1):
    f1_result = f1.result()  # Wait for f1 to be done.
    result = frobnicate(input, f1_result)
    return result

f1 = executor.submit(task1, input)
f2 = executor.submit(task2, input, f1)


Otherwise, is there some reason you can't use multiple callbacks, one
to handle the task's output and one to submit the chained task?


def chain_task2(input, f2):
    f2 = executor.submit(task2, input, f2.result())

f1 = executor.submit(task1, input)
f1.add_done_callback(functools.partial(chain_task2, input))

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