Python多连接下载暂停后恢复，下载无休止

import requests
import threading
import os

class DownloadThread(threading.Thread):
    def __init__(self, url, start, end, file):
        threading.Thread.__init__(self)
        self.url = url
        self.start = start
        self.end = end
        self.file = file

    def run(self):
        headers = {'Range': 'bytes={}-{}'.format(self.start, self.end)}
        response = requests.get(self.url, headers=headers, stream=True)
        with open(self.file, 'r+b') as f:
            f.seek(self.start)
            f.write(response.content)

def download(url, num_threads=4):
    response = requests.head(url)
    file_size = int(response.headers['Content-Length'])
    file_name = url.split('/')[-1]
    if os.path.exists(file_name):
        file_size_downloaded = os.path.getsize(file_name)
    else:
        file_size_downloaded = 0

    chunk_size = file_size // num_threads
    threads = []

    for i in range(num_threads):
        start = i * chunk_size + file_size_downloaded
        end = start + chunk_size - 1 if i < num_threads - 1 else file_size - 1
        thread = DownloadThread(url, start, end, file_name)
        thread.start()
        threads.append(thread)

    for thread in threads:
        thread.join()

    print('Download complete!')

# 示例用法
download('http://example.com/file.zip', num_threads=4)