[New-bugs-announce] [issue16998] Lost updates with multiprocessing.Value

Jens Lechtenboerger report at bugs.python.org
Sat Jan 19 20:13:57 CET 2013

New submission from Jens Lechtenboerger:

Maybe I'm misreading the documentation of multiprocessing.Value and
I thought that access to the value field of Value instances was protected by
locks to avoid lost updates.

Specifically, for multiprocessing.Value(typecode_or_type, *args[, lock]) and
multiprocessing.sharedctypes.Value(typecode_or_type, *args[, lock]) the
documentation states:
> By default the return value is actually a synchronized wrapper for the
> object. [...]
> If lock is True (the default) then a new lock object is created to
> synchronize access to the value. If lock is a Lock or RLock object then that
> will be used to synchronize access to the value. If lock is False then
> access to the returned object will not be automatically protected by a lock,
> so it will not necessarily be “process-safe”.

(By the way, I'm not sure why both, multiprocessing.Value and
multiprocessing.sharedctypes.Value are documented.  They appear to be the same

The following tests (also attached as file) show that lost updates may occur
if several instances of multiprocessing.Process increment the same Value that
is passed as args parameter.

def do_inc(integer):
    """Increment integer.value for multiprocessing.Value integer."""
    integer.value += 1

def do_test(notasks):
    """Create notasks processes, each incrementing the same Value.

    As the Value is initialized to 0, its final value is expected to be
    tasks = list()
    integer = multiprocessing.sharedctypes.Value("i", 0)
    for run in range(notasks):
        proc = multiprocessing.Process(target=do_inc, args=(integer,))
    for proc in tasks:
    if integer.value != notasks:
            "Unexpected value: %d (expected: %d)", integer.value, notasks)

if __name__ == "__main__":

Sample invocations and results:

Note that on a single CPU machine the error is not reported for every
execution but only for about every third run.
$ python --version
Python 2.6.5
$ uname -a
Linux ubuntu-desktop #2 Fri Jun 18 20:30:49 CEST 2010 i686 GNU/Linux
$ python test_multiprocessing.py
ERROR:root:Unexpected value: 99 (expected: 100)

On a quadcore, the error occurs almost every time.
$ uname -a
Linux PC #4 SMP Tue Dec 20 15:22:02 CET 2011 x86_64 GNU/Linux
$ ~/local/Python-2.7.3/python test_multiprocessing.py
ERROR:root:Unexpected value: 95 (expected: 100)
$ ~/local/Python-3.3.0/python test_multiprocessing.py
ERROR:root:Unexpected value: 86 (expected: 100)

components: Library (Lib)
files: test_multiprocessing.py
messages: 180251
nosy: lechten
priority: normal
severity: normal
status: open
title: Lost updates with multiprocessing.Value
type: behavior
versions: Python 3.3
Added file: http://bugs.python.org/file28782/test_multiprocessing.py

Python tracker <report at bugs.python.org>

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