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POAppending a process to a list (but doing nothing with it) alters program behaviour
 

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  1. POAppending a process to a list (but doing nothing with it) alters program behaviour
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    <p>In the following program, when I append the process to a list (a seemingly pointless thing to do), it runs as expected. But if I remove the append, the processes destructor is called many times before it is even run. There are only <code>n</code> constructions but <code>(n)(n+1)/2</code> (where <code>n</code> is the number of processes) destructions. This leads me to believe that each process is being copied into each new process and then immediately destroyed. Perhaps that is how the multiprocessing module works. It makes sense, since each process is a fork of the current one. But, what is the significance of appending to the list? Why does simply doing that stop this behaviour?</p> <p>Here's the test and sample output:</p> <pre><code>import multiprocessing class _ProcSTOP: pass class Proc(multiprocessing.Process): def __init__(self, q, s): s.i += 1 self._i = s.i self._q = q print('constructing:', s.i) super().__init__() def run(self): dat = self._q.get() while not dat is _ProcSTOP: self._q.task_done() dat = self._q.get() self._q.task_done() print('returning: ', self._i) def __del__(self): print('destroying: ', self._i) if __name__ == '__main__': q = multiprocessing.JoinableQueue() s = multiprocessing.Manager().Namespace() s.i = 0 pool = [] for i in range(4): p = Proc(q, s) p.start() pool.append(p) # This is the line to comment for i in range(10000): q.put(i) q.join() for i in range(4): q.put(_ProcSTOP) q.join() print('== complete') </code></pre> <hr> <p>Sample output with append:</p> <pre><code>constructing: 1 constructing: 2 constructing: 3 constructing: 4 returning: 3 returning: 2 == complete returning: 1 returning: 4 destroying: 4 destroying: 3 destroying: 2 destroying: 1 </code></pre> <hr> <p>Sample output without append:</p> <pre><code>constructing: 1 constructing: 2 constructing: 3 destroying: 1 constructing: 4 destroying: 1 destroying: 2 destroying: 3 destroying: 1 destroying: 2 returning: 1 returning: 4 returning: 2 == complete returning: 3 destroying: 1 destroying: 3 destroying: 2 destroying: 4 </code></pre>
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    1. UStjm
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      1. POAppending a process to a list (but doing nothing with it) alters program behaviour
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      1. VTUpMod
    2. VO
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      1. VTUpMod
    1. CODo you suppose it has anything to do with garbage collection? The symptoms indicate that it is necessary to keep a reference to the class instance.
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      1. USDave
    2. CO@Dave. That's a good point. I suspect you're on the right track there.
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      1. UStjm
 

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