My vote is for making this a ValueError in both cases (and amending the doc appropriately as well).  The error amounts to the same thing: the module did not have loader (implicitly or explicitly).  If someone wants to distinguish between the two they can explicitly check __loader__ on the module.

Incidently, _find_and_load_unlocked() has similar code to find_loader() (Lib/importlib/_bootstrap:1523) and raises ImportError instead of ValueError.  That's actually fine since it's a different situation.  However, _find_module() does not handle when __loader__ does not exist, so you would get neither ValueError nor ImportError.  I expect we'd want it or _find_and_load_unlocked() to convert the AttributeError into ImportError to be consistent both with the fix for this issue and with how we handle the __loader__ == None case there.

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For reference, here is the original python-dev thread:

  http://mail.python.org/pipermail/python-dev/2013-January/123777.html

The reference to ValueError is in the importlib docs:

  http://docs.python.org/dev/library/importlib.html#importlib.find_loader