Davin Potts added the comment:

Please keep in mind that this issue should only be encountered by people using 
Python 3.x on a platform like Hurd (an unsupported platform) that has no 
working sem_open implementation.  If we try to imagine what Python 3.x users 
should expect, it's that they should see consistent behavior when importing or 
trying to use various pieces of multiprocessing on such an incomplete system.  
Or we should imagine Python 2.x users attempting to adopt Python 3.x and 
choosing to do so on such an incomplete system -- again, consistency is to be 
expected or at least desperately wished for.  It is reasonable to believe that 
it is the exception that people like Ole have even seen this behavior and their 
reaction is that of a request for consistency.  This is worth fixing.


Adding the "from .synchronize import SEM_VALUE_MAX as maxsize" as was suggested 
would indeed trigger the ImportError but it would also trigger this ImportError 
immediately upon simply doing an "import multiprocessing" which is not the 
current behavior on systems like Hurd.  This would effectively cut off those 
Python 3.x users from other functionality in multiprocessing which does not 
depend upon a working sem_open implementation.

It'd make for a nice, tidy solution, that "from .synchronize ..." -- but I 
wasn't prepared to alter other expected behaviors.  Hence the earlier comment 
about, "I believe a less-than-simple refactoring is required to get both 
execution paths to encounter the same exception".


If we don't do this for Hurd users (they need love too), we should do it for 
users like Ole who sometimes find themselves working on a cool package like 
astropy on a system like Hurd.

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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue23400>
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