我的代码有两个问题：
1.我在这里找到了第一个问题的解决方案。https://stackoverflow.com/a/25165183/14900791：
Lock()函数创建了一个全新的锁--只有调用该函数的线程才能使用，这就是它不起作用的原因，因为每个线程都锁定了一个完全不同的锁。

1. Mixdrop（mxdcontent.net）只允许两个视频在同一个IP，所以代码只适用于两个部分，其他得到状态代码509（我没有检查状态代码，所以我没有得到一个错误）.

import requests
import threading
from tqdm import tqdm

DOWNLOAD_CHUNK_SIZE = 1 << 20 # 1 MiB

# global lock instance
lock = threading.Lock()

class DownloadPart:
    def __init__(self, url, byte_range) -> None:
        self.url = url
        self.byte_range = byte_range

    def download(self, file, pbar=None):
        response = requests.get(
            self.url,
            headers={"Range": "bytes={}-{}".format(*self.byte_range)},
            allow_redirects=True,
            stream=True,
        )

        written = 0

        for chunk in response.iter_content(chunk_size=DOWNLOAD_CHUNK_SIZE):
            if chunk:
                lock.acquire()

                file.seek(self.byte_range[0] + written)
                length = file.write(chunk)
                file.flush()

                written += length
                pbar.update(length)

                lock.release()

class Downloader:
    def __init__(self, url, parts=10):
        self.url = url
        self.parts = parts

    def _get_file_size(self) -> int:
        info = requests.head(self.url, allow_redirects=True)

        info.raise_for_status()

        size = info.headers.get("content-length", None)
        assert size
        return int(size)

    def download(self, filename):
        file_size = self._get_file_size()
        # file_size = 1024

        size_per_part = file_size // self.parts

        print(file_size, size_per_part)

        file = open(filename, "wb")

        pbar = tqdm(total=file_size)

        threads = []
        for index in range(self.parts):
            # fix last part have more bytes
            if index + 1 == self.parts:
                byte_range = (size_per_part * index, file_size - 1)
            else:
                byte_range = (size_per_part * index, size_per_part * (index + 1) - 1)

            thread = threading.Thread(
                target=DownloadPart(self.url, byte_range).download, args=(file,), kwargs={"pbar": pbar}
            )
            thread.start()
            threads.append(thread)

        for thread in threads:
            thread.join()

        file.close()

URL = "https://s-delivery38.mxdcontent.net/v/8a5f59673042ed97c402be84ceeb20d9.mp4?s=TfiDzO2oBLrhub_GhToCiQ&e=1676489987&_t=1676476332"

d = Downloader(URL)

d.download("video.mp4")