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Currently, when you create a Process, all arguments you pass to its __init__ get pickled. I understood this is done because arguments to __init__ almost always become attributes to the Process. Like this:
def MyProcess(multiprocessing.Process):
 def __init__(self, whatever):
 self.whatever = whatever
Of course, attributes must be pickled so they can be accessed from the separate process (on Windows). And indeed in most cases all arguments to __init__ become attributes, so this makes sense.
But, in some cases you pass in arguments to __init__ that do not become attributes. In my case, __init__ takes an object, and takes some attributes of this object as attributes to itself. The object is unpicklable, but the attributes are. So I had to make some ugly workaround to make the program run.
So I think it would be better if Process would be smart enough to pickle only the arguments that get set as attributes.

Sorry, I accidentally typed "def" instead of "class" in my code sample.

As long as you don't pass the arguments on to Process.__init__() when you call it there should be no problem.
The following program works, but will fail with RuntimeError if you uncomment the comment line:
from multiprocessing import Process
class Unpicklable(object):
 def __reduce__(self):
 raise RuntimeError
class MyProcess(Process):
 def __init__(self, foo, unpicklable_bar):
 Process.__init__(self,
 #args=(foo, unpicklable_bar)
 )
 self.foo = foo
 self.baz = str(unpicklable_bar)
 def run(self):
 print(self.foo)
 print(self.baz)
if __name__ == '__main__':
 p = MyProcess([1,2,3], Unpicklable())
 p.start()
 p.join()

I don't think there is any problem here since you have control over which arguments you pass to __init__.
Without a reason why that is not a solution I will eventually close the issue as rejected.

I opened this issue 2 years ago, and I don't remember it being easily solvable back then. But I've long forgotten what the problems were, and I've lost personal interest in it, so I guess we'll just let it go.

OK, I'll close.