[issue17914] add os.cpu_count()

[Ezio Melotti]

Ezio Melotti added the comment:

Returning None sounds reasonable to me.
Raising an exception pretty much means that the function should always be called in a try/except (unless you are sure that the code is running on an OS that knows the number of CPUs).  Returning -1 is not very Pythonic, and between 0 and None I prefer the latter, since it's IMHO a clearer indication that the value couldn't be determined.

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nosy: +ezio.melotti

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<http://bugs.python.org/issue17914>
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