test_importlib.util provides the test_both() function that facilitates testing both the frozen and source versions of importlib.  The function helps to keep the tests more maintainable.  However, the following test modules are not using test_both():

Lib/test/test_importlib/test_abc.py
Lib/test/test_importlib/test_api.py
Lib/test/test_importlib/test_locks.py
Lib/test/test_importlib/test_spec.py
Lib/test/test_importlib/test_windows.py

Furthermore, the remaining test modules use test_both() inconsistently.  Here is a patch that makes consistent use of test_both() and formats usage in a way that helps with readability.

I see you noticed my shift in strategy after I realized part way through a cleaner way of doing things. =)

LGTM

I don't love the formatting of the test_both() lines, but I think that one is just an aesthetic thing that will never make people happy -- weird line wrapping or really long lines -- so just leave it as-is in your patch.

> I don't love the formatting of the test_both() lines, but I think that one is just an aesthetic thing that will never make people happy -- weird line wrapping or really long lines -- so just leave it as-is in your patch.

Yeah, I went with the formatting that I did because I found it a
little easier to follow (stronger separation between elements) and
visually distinguish in the code.  It was also a little easier to
grep/search for.

FWIW, this change was motivated by the importlib backport (I found some time to work on it).  The import_importlib()/test_both() approach definitely makes backporting the tests easier (thanks for that).

BTW, thanks for also consolidating the various test_importlib util modules.  That also helped.

New changeset 34d65746d5ca by Eric Snow in branch 'default':
Issue #21503: Use test_both() consistently in test_importlib.
http://hg.python.org/cpython/rev/34d65746d5ca