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I'm trying to remove all uses of imp.find_module()/load_module() and multiprocessing seems to have a single use of both purely for (re)loading a module. The attached patch moves over to importlib.find_loader() and subsequent load_module() call to match the semantics of imp.find_module()/load_module(). If a guaranteed reload is not necessary then importlib.import_module() is a single-line change.
I ran the test suite, but there don't seem to be any explicit tests for this chunk of code (or am I missing something?).

I think this change will potentially make the main module get imported twice under different names when we transfer pickled data between processes. The current code (which is rather a mess) goes out of its way to avoid that.
Basically the main process makes sys.modules['__mp_main__'] an alias for the main module, and other process import the parent's main module with __name__ == '__mp_main__' and make sys.modules['__main__'] an alias for that. This means that any functions/classes defined in the main module (from whatever process) will have
 obj.__module__ in {'__main__', '__mp_main__'}
Unpickling such an object will succeed in any process without reimporting the main module.
Attached is an alternative patch which is more like the original code and seems to work. (Maybe modifying loader.name is an abuse of the API.)

It is an abuse since I didn't design that part of the API to function that way, but it's cool that it just happens to. =)
I do see your use-case and it is legitimate, although extremely rare and narrow. Let me think about whether I want to add specific support either through your approach, Richard, or if I want to decouple the setting of module attributes so that it is more along the lines of::
 main_module = imp.new_module('__mp_main__')
 loader.set_attributes(main_module) # BRAND-NEW; maybe private to the stdlib?
 main_module.__name__ = '__mp_main__'
 code_object = loader.get_code(main_name)
 sys.modules['__main__'] = sys.modules['__mp_main__'] = main_module # OLD
 exec(code_object, main_module.__dict__)
I'm currently leaning towards the latter option since it's an annoying bit to get right and it doesn't hurt anything to expose.

So I think I have come up with a way to expose a new method that makes this use-case doable and in a sane manner. Richard, let me know what you think so that I know that this makes sense before I commit myself to the new method (init_module_attrs())::
--- a/Lib/multiprocessing/forking.py	Fri May 24 13:51:21 2013 +0200
+++ b/Lib/multiprocessing/forking.py	Fri May 24 08:06:17 2013 -0400
@@ -449,7 +449,7 @@
 elif main_name != 'ipython':
 # Main modules not actually called __main__.py may
 # contain additional code that should still be executed
- import imp
+ import importlib
 
 if main_path is None:
 dirs = None
@@ -460,16 +460,17 @@
 
 assert main_name not in sys.modules, main_name
 sys.modules.pop('__mp_main__', None)
- file, path_name, etc = imp.find_module(main_name, dirs)
+ # We should not try to load __main__
+ # since that would execute 'if __name__ == "__main__"'
+ # clauses, potentially causing a psuedo fork bomb.
+ loader = importlib.find_loader(main_name, path=dirs)
+ main_module = imp.new_module(main_name)
 try:
- # We should not do 'imp.load_module("__main__", ...)'
- # since that would execute 'if __name__ == "__main__"'
- # clauses, potentially causing a psuedo fork bomb.
- main_module = imp.load_module(
- '__mp_main__', file, path_name, etc
- )
- finally:
- if file:
- file.close()
+ loader.init_module_attrs(main_module)
+ except AttributeError:
+ pass
+ main_module.__name__ = '__mp_main__'
+ code = loader.get_code(main_name)
+ exec(code, main_module.__dict__)
 
 sys.modules['__main__'] = sys.modules['__mp_main__'] = main_module

Looks good to me.
(Any particular reason for ignoring AttributeError?)

Catching the AttributeError is in case a loader is used that doesn't define init_module_attrs(). Since it will be new to Python 3.4 I can't guarantee that it will exist (in case someone doesn't subclass the proper ABC).

The unit tests pass with the patch already (if we don't delete the "import imp" line).
What attributes will be set by init_module_attrs()?

In the common case of SourceLoader it will set __loader__, __package__, __file__, and __cached__.

New changeset 97adaa820353 by Brett Cannon in branch 'default':
Issue #17314: Stop using imp in multiprocessing.forking and move over
http://hg.python.org/cpython/rev/97adaa820353