如何将队列字典传递给 multiprocessing.Process

Question

I am not sure if it is possible to pass an dictionary of Queues as argument to a Process. If it is only possible as kwargs, how can it be done?

from multiprocessing import Process, Queue


class WordsManager:

    def __init__(self, my_dict):
        self.dict = my_dict

    def run(self):
        pass


def words_worker(my_dict):
    worker = WordsManager(my_dict)
    worker.run()


def start_job_manager():
    my_dict = {}
    for language in ('en', 'de', 'es'):
        my_dict[language] = Queue()

    words_manager = {'process': None, 'my_dict': my_dict}
    words_manager["process"] = Process(target=words_worker, args=(words_manager['my_dict']))
    words_manager["process"].daemon = True
    words_manager["process"].start()

    return words_manager


start_job_manager()

The error is:

Process Process-4:
Traceback (most recent call last):
   File "/home/antonio/python/lib/python3.4/multiprocessing/process.py", line 254, in _bootstrap
self.run()
  File "/home/antonio/python/lib/python3.4/multiprocessing/process.py", line 93, in run
self._target(*self._args, **self._kwargs)
TypeError: words_worker() takes 1 positional argument but 3 were given

Answer 1

The problem is in the arguments you pass to the process:

args=(words_manager['my_dict'])

The braces are not interpreted as tuple, so you're not passing a sequence of arguments to the args . Instead you should explicitly create a 1-element tuple by placing , at the end:

args=(words_manager['my_dict'],)

From the Python Docs :

A special problem is the construction of tuples containing 0 or 1 items: the syntax has some extra quirks to accommodate these. Empty tuples are constructed by an empty pair of parentheses; a tuple with one item is constructed by following a value with a comma (it is not sufficient to enclose a single value in parentheses). Ugly, but effective.

Answer 2

If I use a comma after the element, it runs properly

args=(words_manager['my_dict'],)

I suppouse that without the comma, Python interprets each value from the dictionary as an argument, but with the comma it is treated as a tuple with words_manager['my_dict'] as a sole argument.