Ok -- made some progress.  Built a unit test -- it's similar to the doctest from memmap.py, but rewritten in py.test format (uses BaseNumpyAppTest from pypy.module.micronumpy.test.test_base, just like test_numeric.py does).

Here is the next problem I face:  To run my unit test, I added in an import of my new memmap.py, which I have placed in lib_pypy/numpypy/core/memmap.py, and linked into core/__init__.py.  This module imports mmap.py for obvious reasons (it wants to use mmap.py to create the mmap object).  That import fails when I run the test: python2.7 pytest.py pypy/module/test_lib_pypy/numpypy/core/test_memmap.py ("Import error: no module named mmap")

It fails because it can not import the module mmap.  Whereas if I just fire up a normal interpreter-level pypy I have no problem importing mmap.  Clearly it does exist in pypy. 

I don't understand which modules will be loadable and which modules will not be loadable when I run in pytest.py.  Somehow, clearly, the BaseNumpyAppTest inheritance mechanism has allowed all the numpypy stuff to show up (but only inside the class definition, not at module scope), but other pypy application-level modules are not available.  I need to somehow invoke the same magic for mmap if I want to be able to use the mmap module to implement the numpypy memmap functionality.    I read coding-guide.rst but it's still not obvious to me.  (Alternatively I suppose the answer could be full translation, but testing this way would take a full 90-minute translation every time I wanted to change a line of code.)

If you feel this detailed emailing is wasting your time, just let me know and I can drop it.   (Or if you want to take the conversation off pypy-dev that's ok too.)