Multiprocessing Manager on 3D list - no change of the list possible

[John81]

BPO	32538
Nosy	@pitrou, @berkerpeksag, @applio

^{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 2019-03-31.17:37:10.941>
created_at = <Date 2018-01-12.15:05:56.625>
labels = ['invalid', 'type-bug', 'library']
title = 'Multiprocessing Manager on 3D list - no change of the list possible'
updated_at = <Date 2019-03-31.17:37:27.677>
user = 'https://bugs.python.org/John81'

bugs.python.org fields:

activity = <Date 2019-03-31.17:37:27.677>
actor = 'berker.peksag'
assignee = 'none'
closed = True
closed_date = <Date 2019-03-31.17:37:10.941>
closer = 'berker.peksag'
components = ['Library (Lib)']
creation = <Date 2018-01-12.15:05:56.625>
creator = 'John_81'
dependencies = []
files = []
hgrepos = []
issue_num = 32538
keywords = []
message_count = 2.0
messages = ['309858', '339273']
nosy_count = 4.0
nosy_names = ['pitrou', 'berker.peksag', 'davin', 'John_81']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue32538'
versions = ['Python 3.6']

[John81]

I have the following code, which works without multiprocessing:

data=[[['','','','','','','','','','','','']]]
data[0][0][0] = 5
data[0][0][1] = "5" # data in the array is mixed with float and str
print(data)

#=> [[[5, '5', '', '', '', '', '', '', '', '', '', '']]]

Now I want to use Multiprocessing and every process should be able to change the 3D list. This doesn't work and no error message is shown.

from multiprocessing import Process, Manager
manager=Manager()
data=manager.list([[['','','','','','','','','','','','']]])
data[0][0][0] = 5
data[0][0][1] = "5"
print(data)    

#=> [[['', '', '', '', '', '', '', '', '', '', '', '']]]

I found the following text:

list(sequence)
Create a shared list object and return a proxy for it.
Changed in version 3.6: Shared objects are capable of being nested. For     
example, a shared container object such as a shared list can contain 
other shared objects which will all be managed and synchronized by the 
SyncManager.(https://docs.python.org/3/library/multiprocessing.html)

Unfortunately it also doesn't work with 3.6.3, same problem as before! But as it should work, I guess it's a bug?

I use Ubuntu 16.04...

[berkerpeksag]

I'm a bit late to reply, but I think you are using the API wrong. Please try the following snippet:

from multiprocessing import Manager

manager = Manager()

data = manager.list([manager.list([manager.list(['', '', '', '', '', '', '', '', '', '', '', ''])])])

print(data[0][0])
print(data[0][0][0])
print(data[0][0][1])

data[0][0][0] = 5
data[0][0][1] = '5'

print(data[0][0])
print(data[0][0][0])
print(data[0][0][1])

Output:

$ ./python.exe example.py
['', '', '', '', '', '', '', '', '', '', '', '']

[5, '5', '', '', '', '', '', '', '', '', '', '']
5
5

You can also take a look at the tests in the original commit that implemented this feature:

86a7668