Davin Potts added the comment:
It looks like the first 'Called Foo.__new__' is being reported by the child
(pool of 1) process and the second 'Called Foo.__new__' is being reported by
the parent process. In multiprocessing, because objects are by default
serialized using pickle, this may be caused by the unpickling of the Foo object
by the parent process which is something you would not experience when using
ThreadPool because it does not have the same need for serialization.
Example showing invocation of __new__ as part of unpickling:
>>> class Foo(object):
... def __new__(cls):
... print("New")
... return object.__new__(cls)
...
>>> import pickle
>>> f = Foo()
New
>>> pf = pickle.dumps(f, protocol=2)
>>> pickle.loads(pf) # unpickling triggers __new__
New
<__main__.Foo object at 0x1084a06d0>
Having discovered this phenomenon, is this causing a problem for you somewhere
in code? (Your example code on github was helpful, thank you, but it didn't
merely demonstrated the behavior and didn't show where this was causing you
pain.)
----------
nosy: +davin
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Python tracker <rep...@bugs.python.org>
<http://bugs.python.org/issue30018>
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