E.g. _frozen_importlib, builtins, signal.

Should probably check the state of sys.modules in importlib._bootstrap._setup() and see if the missing modules are there and then just retroactively set __loader__ to BuiltinImporter (as is already done in that function for sys and _imp): http://hg.python.org/cpython/file/905b4e3cf6d0/Lib/importlib/_bootstrap.py#l1709 .

All cases but signal can be fixed by importlib._bootstrap._setup() by simply iterating over sys.modules and setting __loader__. Nice and simple. Thanks, abstraction!

The problem is signal who gives the finger to abstraction. That module gets imported directly by C code which cheats by calling PyInit_signal() directly and then calling _PyImport_FixupBuiltin(). I tried setting __loader__ in there but I'm actually triggering a segfault in PyErr_Format()! This is probably because I/O streams are not set up yet at this point in interpreter startup. Anyway, I'm trying to simply not have the code cheat and instead just import the signal module through the proper abstractions.

New changeset 05747d3bcd9c by Brett Cannon in branch '3.3':
Issue #17098: Make sure every module has __loader__ defined.
http://hg.python.org/cpython/rev/05747d3bcd9c

New changeset 1f1a1b3cc416 by Brett Cannon in branch 'default':
Issue #17098: all modules should have __loader__
http://hg.python.org/cpython/rev/1f1a1b3cc416

OK, so my solution for signal worked. All fixed now. If any other modules pop up with __loader__ not set from now on it's because they cheated on import. =)

Looks like I spoke too soon. I realized I had not written a test so I did it quickly and it turns out that while this is fixed for 3.3 it isn't for 3.4.

I have only tried my code with 3.4; but still get problems with the modules 'builtins' and '_frozenimportlib'.

New changeset 4a4688b865ff by Brett Cannon in branch '3.3':
Add a test for fix of issue #17098
http://hg.python.org/cpython/rev/4a4688b865ff

New changeset 19ea454ccdf7 by Brett Cannon in branch '3.3':
Issue #17098: Be more stringent of setting __loader__ on early imported
http://hg.python.org/cpython/rev/19ea454ccdf7

New changeset 306f066e6a33 by Brett Cannon in branch 'default':
Merge w/ 3.3 more fixes thanks to issue #17098
http://hg.python.org/cpython/rev/306f066e6a33

OK, 3.3 and 3.4 now have tests and verify everything is working fine.

I confirm that the bug is fixed, my test-code works now (a modulefinder using importlib instead of imp to find modules).

Thanks for the very fast fix Brett :-)