[issue11969] Can't launch multiproccessing.Process on methods

[Terry J. Reedy]

Terry J. Reedy <tjreedy at udel.edu> added the comment:

After looking at the doc chapter, I get that 'if __name__' block is needed on Windows.

OK. batch mode with if __name__ block:

testmp.py
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print('Top of Module')
class C:
    def f(s): print('Method C.f')

if __name__ == '__main__':
    print('Start of main block')
    import multiprocessing
    p = multiprocessing.Process(target=C.f, args=(C(),))
    p.start()
    p.join()

command-prompt..\python32> python @misc2\testmp.py

output (similar to that of OP):

Top of Module
Start of main block
Traceback (most recent call last):
  File "C:\Programs\Python32\lib\pickle.py", line 679, in save_global
    klass = getattr(mod, name)
AttributeError: 'module' object has no attribute 'f'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "@misc2\testmp.py", line 9, in <module>
    p.start()
  File "C:\Programs\Python32\lib\multiprocessing\process.py", line 130, in start

    self._popen = Popen(self)
  File "C:\Programs\Python32\lib\multiprocessing\forking.py", line 267, in __init__
    dump(process_obj, to_child, HIGHEST_PROTOCOL)
  File "C:\Programs\Python32\lib\multiprocessing\forking.py", line 190, in dump
    ForkingPickler(file, protocol).dump(obj)
  File "C:\Programs\Python32\lib\pickle.py", line 237, in dump
    self.save(obj)
  File "C:\Programs\Python32\lib\pickle.py", line 344, in save
    self.save_reduce(obj=obj, *rv)
  File "C:\Programs\Python32\lib\pickle.py", line 432, in save_reduce
    save(state)
  File "C:\Programs\Python32\lib\pickle.py", line 299, in save
    f(self, obj) # Call unbound method with explicit self
  File "C:\Programs\Python32\lib\pickle.py", line 623, in save_dict
    self._batch_setitems(obj.items())
  File "C:\Programs\Python32\lib\pickle.py", line 656, in batch_setitems
    save(v)
  File "C:\Programs\Python32\lib\pickle.py", line 299, in save
    f(self, obj) # Call unbound method with explicit self
  File "C:\Programs\Python32\lib\pickle.py", line 683, in save_global
    (obj, module, name))
_pickle.PicklingError: Can't pickle <function f at 0x00B4B4B0>: it's not found as __main__.f

C:\Programs\Python32>Top of Module
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Programs\Python32\lib\multiprocessing\forking.py", line 370, in main
    self = load(from_parent)
EOFError

The essential problem seems to be the attempt to pickle the method attribute of the class as a class attribute of the module.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue11969>
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