[issue22600] In Multiprocessing Queue() doesn't work with list : __.put(list) != __.get()

[Amaury Forgeot d'Arc]

Amaury Forgeot d'Arc added the comment:

solution.put(c) takes a *reference* to the list c.
And then the test() function continues running!

By the time the list is serialized and sent back to the main process (in 
another thread), the test() function has already changed it...

As you noticed, you can use immutable objects instead (string, int), or make a 
copy of the list: solution.put(c.copy())

--
nosy: +amaury.forgeotdarc
resolution:  - not a bug
stage:  - resolved
status: open - closed

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[issue22600] In Multiprocessing Queue() doesn't work with list : __.put(list) != __.get()

[CALMET]

New submission from CALMET:

Hello,
I have to find the right order of a list, for example
try = [0,1,2,3,4,5,6,7,8,9]
try = [0,1,2,3,4,5,6,7,9,8]
try = [0,1,2,3,4,5,6,8,7,9]
and so on... in this example there are factorial(10) = 3,6 million possible 
order to test.
The results with be all the list  'try'  that match with the test.

To go faster
- I use mutiproceesiing
- I define   rightorder = Queue() 
- I define a function test   with a line  rightorder.put(try)
- I wait for all the processes are finished  p.join()
- Then I read all the results in the main programm   rightorder.get()

The problem is that on exactly the same example
- the rightorder.put() found by the process are always good
- but the rightorder.get() in the main programm can be different (randomly).

I first believed that when 2 processes call the function  rightorder.put(try)  
at the same time, the lists ares mixed together in the Queue().

But even when I force cpu_count() to 1 (mono process) I still have the problem.

To solve the problem I converted the list try=[2,3,0,4,2,9,7,8,1,5]
in a string try2=2304297815
and since now  rightorder.put(try)  and  rightorder.get()  always give the same 
results.

So I suppose that in multiprocessing with a Queue() rightorder.put(try)  works 
if try is a number or a string
but doesn't works if try is a list.

--
files: BisesAmouRmulti-debug.py
messages: 228993
nosy: AlainCALMET
priority: normal
severity: normal
status: open
title: In Multiprocessing Queue() doesn't work with list : __.put(list) != 
__.get()
type: behavior
versions: Python 3.3
Added file: http://bugs.python.org/file36865/BisesAmouRmulti-debug.py

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