> 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.

Yes, and I compiled Python with disabled sem_open for testing.

Python not always specifies what exact type of exception is raised. When you 
pass wrong type of argument to a function, it can raise TypeError or 
AttributeError. If you request unsupported feature, it can raise ImportError, 
AttributeError, or even NameError. Python and C implementations can raise 
different errors (C implementation usually more strict). Not all such cases are 
considered as bugs.

> 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.

No, I suggested to replace only the line that produces AttributeError now.