Promise implementation using ThreadExecutor and Future in Python

promise-thread.py

from concurrent.futures import ThreadPoolExecutor, Executor, Future


class Promise:
    def __init__(self, fn, *args, **kwargs):
        self.fn = fn
        self.callbacks = []
        self.last_result = None
        self.executor: Executor = ThreadPoolExecutor(max_workers=1)
        self.future: Future = self.executor.submit(fn, *args, **kwargs)
        self.future.add_done_callback(self._execute_callbacks)

    def then(self, callback):
        if self.future.done() and not self.callbacks:
            result = self.last_result or self.future.result()
            callback(result)
        else:
            self.callbacks.append(callback)
        return self

    def _execute_callbacks(self, future):
        self.last_result = future.result()
        while self.callbacks:
            cb = self.callbacks.pop(0)
            cb_res = cb(self.last_result)
            self.last_result = cb_res or self.last_result
        self.executor.shutdown(wait=False)

    def wait_for_result(self):
        return self.future.result()