> Here's my take.  No one will care about _frozen_importlib vs.
> importlib._bootstrap normally, right?  If __module__/__file__ says
> _frozen_importlib, it's no big deal.

The reason I'd prefer __file__ to point to the actual Python file is so
that people reading a traceback can find the source code. Of course
that's a bit minor.
(and, incidentally, the traceback itself will display the source code
lines)

> The only time you care about the distiction for importlib._bootstrap
> is when you're hacking on _bootstrap.py.  So let's keep the common
> case in sight and go from there.

Agreed.

> For the first part, let's simply ignore the pure Python
> importlib._bootstrap by default?  Then we stick a context manager in
> importlib.test.util that enables it.  When you're hacking on
> _bootstrap.py, you switch it over.  The common path stays pretty
> clean.

Looks good to me.

> I've attached a patch for the first part which has similarities to
> Antoine's.  (I didn't apply the context manager to the importlib test
> cases though.)

I think set_bootstrap() looks a bit fragile, since we have to manually
add any importlib attributes that are exported in importlib/__init__.py.
Perhaps we could detect them automatically?