On 12/21/11 8:11 PM, Andrew Berg wrote:
On 12/21/2011 1:29 PM, Ethan Furman wrote:
Anything that runs at import time should be protected by the `if
__name__ == '__main__'` idiom as the children will import the __main__
module.
So the child imports the parent and runs the spawn code again? That
makes sense.
This is a problem with multiprocessing on Windows. Your code would work fine on
a UNIX system. The problem is that Windows does not have a proper fork() for
multiprocessing to use. Consequently, it has to start a new Python process from
scratch and then *import* the main module such that it can properly locate the
rest of the code to start. If you do not guard the spawning code with a __main__
test, then this import will cause an infinite loop.
Just as a further note on these lines, when older versions of setuptools and
distribute install scripts, they generate stub scripts that do not use a
__main__ guard, so installing a multiprocessing-using application with
setuptools can cause this problem. I believe newer versions of distribute has
this problem fixed, but I'm not sure.
--
Robert Kern
"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco
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