try to race condition

xxx.py

import  concurrent.futures
import unittest
from collections import deque, Counter

import time


class QList(list):
    def __init__(self, seq=()):
        super().__init__(seq)
        self._queue = deque()

    def free(self, index, set_to=None):
        self[index] = set_to
        self._queue.append(index)

    def append(self, p_object):
        if self._queue:
            index = self._queue.popleft()
            self[index] = p_object
        else:
            super().append(p_object)
            time.sleep(0.01)
            index = len(self) - 1
        return index


class Foo(unittest.TestCase):
    def test_append(self):
        fs = []
        q = QList()
        cnt =   10
        workers=2
        with concurrent.futures.ThreadPoolExecutor(max_workers=workers) as pool:
            for x in range(cnt):
                f = pool.submit(q.append, x)
                fs.append(f)

        rets = [f._result for f in fs]
        c = Counter(rets)
        self.assertEqual(cnt, len(c))
        for key, value in c.items():
            self.assertEqual(1, value)