multiprocessing: AttributeError: 'SimpleQueue' object has no attribute '_poll'

[DanielMoore]

BPO	30301
Nosy	@pfmoore, @tjguk, @zware, @serhiy-storchaka, @zooba, @applio, @zhangyangyu
PRs	bpo-30301: Fix AttributeError when using SimpleQueue.empty() under *spawn* and *forkserver* start methods #1601 [3.5] bpo-30301: Fix AttributeError when using SimpleQueue.empty() #1627 [3.6] bpo-30301: Fix AttributeError when using SimpleQueue.empty() #1628

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2017-05-17.14:06:16.744>
created_at = <Date 2017-05-07.19:01:50.828>
labels = ['3.7', 'type-bug', 'library', 'OS-windows']
title = "multiprocessing: AttributeError: 'SimpleQueue' object has no attribute '_poll'"
updated_at = <Date 2017-05-17.14:06:16.743>
user = 'https://bugs.python.org/DanielMoore'

bugs.python.org fields:

activity = <Date 2017-05-17.14:06:16.743>
actor = 'xiang.zhang'
assignee = 'none'
closed = True
closed_date = <Date 2017-05-17.14:06:16.744>
closer = 'xiang.zhang'
components = ['Library (Lib)', 'Windows']
creation = <Date 2017-05-07.19:01:50.828>
creator = 'Daniel Moore'
dependencies = []
files = []
hgrepos = []
issue_num = 30301
keywords = []
message_count = 7.0
messages = ['293209', '293753', '293774', '293838', '293850', '293853', '293854']
nosy_count = 8.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'serhiy.storchaka', 'steve.dower', 'davin', 'xiang.zhang', 'Daniel Moore']
pr_nums = ['1601', '1627', '1628']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30301'
versions = ['Python 3.5', 'Python 3.6', 'Python 3.7']

[DanielMoore]

I originally posted this as a question on StackOverflow thinking I was doing something wrong:

http://stackoverflow.com/questions/43834494/python-3-6-attributeerror-simplequeue-object-has-no-attribute-poll/43835368#43835368

But I think I found the solution and answered my own question. I'm pretty sure you need to set:

self._poll = self._reader.poll

in the __setstate__ method in the SimpleQueue class of queues.py from the multiprocessing library. Otherwise, I'd love to know an alternative solution.

Thanks!

[zhangyangyu]

Related commit is bdb1cf1. A simple reproduce snippet:

import multiprocessing as mp

def foo(q):
    q.put('hello')
    assert not q.empty()

if __name__ == '__main__':
    mp.set_start_method('spawn')
    q = mp.SimpleQueue()
    p = mp.Process(target=foo, args=(q,))
    p.start()
    print(q.get())
    p.join()

[serhiy-storchaka]

LGTM. But can you convert the reproducer to a test?

[zhangyangyu]

Thanks for your reply Serhiy. Test added. :-)

[zhangyangyu]

New changeset 6f75bc0 by Xiang Zhang in branch 'master':
bpo-30301: Fix AttributeError when using SimpleQueue.empty() (bpo-1601)
6f75bc0

[zhangyangyu]

New changeset 9081b36 by Xiang Zhang in branch '3.5':
bpo-30301: Fix AttributeError when using SimpleQueue.empty() (bpo-1601) (bpo-1627)
9081b36

[zhangyangyu]

New changeset 43d4c03 by Xiang Zhang in branch '3.6':
bpo-30301: Fix AttributeError when using SimpleQueue.empty() (bpo-1601) (bpo-1628)
43d4c03