New submission from Graham Cummins :
When attempting to pass instances of a custom class in the argument list
of a multiprocessing.Pool.map call, I encounter an inexplicable AttributeError.
A simplified test script that creates the bug is included. It fails for both
Python 2.7.1 and 3.2 on my 4-core 64 bit linux machine.
Briefly, the code creates a subclass of "dict" that checks for a "_ro"
attribute before handling a "__setitem__". Empty instances of this class
can be passed to Pool.map fine, but if they have any content, I get
that multiprocessing/queues.py, line 378 ("recv()"), calls my custom
__setitem__, which fails, due to a missing _ro attribute.
This is very confusing. For one thing, much of the Traceback seems
missing. recv() is not directly a call to my customized __setitem__,
and I can't thing why it should ever need to invoke such a call
For another, the custom class _does_ set the _ro attribute, on __init__.
Testing for this attribute, or calling __setitem__, succeeds in every other
context I've tested, so it seems that the instance that is
being used in the multiprocessing context is not the same underlying object
that I pass in.
--
components: Library (Lib)
files: bug.py
messages: 137373
nosy: Graham.Cummins
priority: normal
severity: normal
status: open
title: multiprocessing.Pool initialization seems to call methods it should not
(and on badly formed objects)
versions: Python 2.7
Added file: http://bugs.python.org/file22211/bug.py
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