submit_f_test.py

import concurrent.futures
import time


class Tree:
    def __init__(self, left=None, right=None):
        self.left = left
        self.right = right


executor = concurrent.futures.ThreadPoolExecutor(max_workers=3)


def count_tree(future, tree):
    time.sleep(1)
    left_counted = right_counted = False
    count = 1

    def _set_result():
        if left_counted and right_counted:
            future.set_result(count)

    if tree.left:

        def _left_result(f):
            nonlocal count, left_counted
            left_counted = True
            count += f.result()
            _set_result()

        f = executor.submit_f(count_tree, tree.left)
        f.add_done_callback(_left_result)
    else:
        left_counted = True

    if tree.right:
        right = None

        def _right_result(f):
            nonlocal count, right_counted
            right_counted = True
            count += f.result()
            _set_result()
        f = executor.submit_f(count_tree, tree.right)
        f.add_done_callback(_right_result)
    else:
        right_counted = True

    _set_result()


t = Tree(left=Tree(left=Tree(left=Tree()), right=Tree()))
f = executor.submit_f(count_tree, t)
print(f.result())