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This is closely related to issue #19697.

D'oh, I forgot there was a runpy API change I planned to offer as part of the PEP 451 integration: exposing a "target" parameter in both run_path and run_module.
http://www.python.org/dev/peps/pep-0451/#the-target-parameter-of-find-spec
I guess that slips to 3.5 now, since there's no way it will be in for feature freeze :(

_run_module_as_main() is particularly related to #19697.

OK, I just noticed that importlib.find_spec copies the annoying-as-hell behaviour of importlib.find_loader where it doesn't work with dotted names. Can we fix that please? It's stupid that pkgutil.get_loader has to exist to workaround that design flaw for find_loader, and I'd hate to see it replicated in a new public facing API.

Deleted a bunch of code, and runpy now correctly sets both __file__ and __cached__ (runpy previously never set the latter properly).
You can also see the reason for my rant above in the form of runpy._fixed_find_spec. importlib.find_loader was always kind of useless in end user code that needed to handle arbitrary modules, you needed to use the pkgutil.get_loader wrapper instead. It would be nice if importlib.find_spec "just worked", instead of only working for top level modules. At the very least, it should fail noisily if a dotted name is passed in without specifying a path argument.
I may have argued in favour of the side effect free find_loader in the past, if I have, consider this me admitting I was wrong after wasting a bunch of time hunting down unexpected import errors in test_runpy after the initial conversion to using find_spec.
There's one final change needed to address the pickle-in-main compatibility issue: runpy has to alias the module under its real name as well as __main__. Once it does that, then using __spec__.name (when available) should ensure pickle does the right thing.

Can you file a separate bug, Nick, for the importlib.find_spec() change you are after? I don't want to lose track of it (and I get what you are after but we should discuss why importlib.find_loader() was designed the way it was initially).

Issue 19944 now covers moving the current importlib.find_spec to importlib.find_spec_on_path and having importlib.find_spec behave like runpy._fixed_find_spec in my patch.

patch LGTM

I added some unit tests for the interactions between runpy and namespace packages, which showed that I was doing the check for __main__ submodules and the check for "no loader" in the wrong order.
Last missing piece is to ensure that __spec__ is being populated appropriately, then I'll check this in.

So close!
importlib.util.spec_from_file_location failed me (unsurprisingly) when it came to the zipfile execution tests. I'm thinking I'll just hack in a way to avoid checking the loader when the expected loader is set to None.

Latest version simply doesn't check the loader type for the zipimport tests. (Note that just using find_spec doesn't work, since the directory and zipfile execution tests include implicit sys.path manipulation)
I also removed the separated "precompiled" flag that was in earlier patches, since importlib.util.spec_from_file_location deals with that for us.
If the full regression test suite passes, I'll be checking this version in.

Working on the docs updates made me realise the test cases didn't cover the "no suffix" case that is causing grief in issue 19946.
So I've added a test case for that now, but haven't fixed it yet (will need to deal with the __spec__ = None case for such scripts)

Final patch that reflects the version I'm about to commit. It includes appropriate docs updates, and ensures __main__.__spec__ is None in the cases where the import system isn't involved in initialising main.

New changeset 51dddfead80a by Nick Coghlan in branch 'default':
Issue #19700: set __spec__ appropriately in runpy
http://hg.python.org/cpython/rev/51dddfead80a 

Final review of the patch showed it *wasn't* quite done, it's still missing the "both __name__ and __spec__.name are configured in sys.modules" change that is needed to get more pickle friendly behaviour from __main__.
However, I wanted to commit this version to help unblock issue 19946.

On second thoughts, I'm going to close this one - if further runpy changes are needed to resolve issue 19702 (using __spec__.name for pickle when appropriate), let's deal with them there.

Thanks for working this out, Nick!