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The pkgutil.ImpImporter documentation says that if dirname is None, ImpImporter(dirname) should create a PEP 302 importer that searches the current sys.path, plus any modules that are frozen or built-in:
http://docs.python.org/dev/library/pkgutil.html#pkgutil.ImpImporter
However, the iter_modules() method of an ImpImporter instance doesn't search sys.path if dirname is None. It returns a generator that always yields nothing. For example--
Python 3.3.0b1 (default:5d43154d68a8, Jul 8 2012, 13:54:45) 
[GCC 4.2.1 Compatible Apple Clang 3.1 (tags/Apple/clang-318.0.58)] on darwin
>>> from pkgutil import ImpImporter
>>> importer = ImpImporter()
>>> list(importer.iter_modules())
[]
Strictly speaking, one could say the documentation only applies to the find_module() method since that's the only method covered by the PEP 302 API However, iter_modules() is a public method. So I think that either iter_modules() should be fixed, made private, or else the documentation clarified by saying that searching sys.path does not apply to iter_modules().
I'm pretty sure though that iter_modules() should be fixed. This is because there are other functions in pkgutil that seem not to work because ImpImporter.iter_modules() behaves the way it does (specifically calling pkgutil.iter_modules() with path=None).

I'm working on a test for this.

Add failing tests.

In addition, before changing ImpImporter.iter_modules(), we should probably also add some tests of the method for values of self.path that do work (i.e. for when self.path is not None). The method is currently untested.
I can work on adding tests for that over the next few days.

The pkgutil import emulation is being deprecated in 3.3
However, I'll use this as useful input for any new walk_packages tests added while resolving #15343