使用多处理“ValueError：需要多于1个值才能解压缩”

Question

我有这段代码：

def characterize_file(filename):
    try:
        # …
        return (filename, characterization)
    except Exception as e:
        print(filename)
        traceback.print_exc()
        raise e

def write_characterization(args):
    # …
    p = multiprocessing.Pool(3)
    characterizations = p.imap(characterize_file, args)
    for (i, (filename, characterization)) in enumerate(characterizations):
        pass # …

有时，我得到这个例外，没有任何其他输出：

  File "/home/progval/etudes/ens/image/dm/venv/local/lib/python2.7/site-packages/mogulex/main.py", line 52, in write_characterization
    for (i, (filename, characterization)) in enumerate(characterizations):
  File "/usr/lib/python2.7/multiprocessing/pool.py", line 659, in next
    raise value
ValueError: need more than 1 value to unpack

它可能来自哪里？ try语句不应该捕获任何错误并打印出来吗？