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These three things do not mix:
 - AttributeError
 - Threads
 - Object methods
An unhandled AttributeError thrown in a thread will not call sys.excepthook if the thread's start function is a class/object method.
Test case:
import sys
import thread
class Dummy:
	def worker(self):
		raise AttributeError
thread.start_new_thread(Dummy().worker, ())
sys.stdin.readline()
Note that you do not get a traceback here. Throwing any other exception type works fine, as does having worker() be a simple function.
I think I've traced the issue to Objects/classobject.c:instance_repr(). It tries to look up the method, making sure to handle any AttributeError this might cause. But it fails to save and restore and Exception currently already active, effectively clearing out the current exception.

If Dummy is a new-style class, the traceback is correctly printed.

Indeed. I assume old style classes are still supported though?

New changeset 8609d7fcdcc7 by Benjamin Peterson in branch '2.7':
prevent writing to stderr from messing up the exception state (closes #14474)
http://hg.python.org/cpython/rev/8609d7fcdcc7 

New changeset 60ad83716733 by Benjamin Peterson in branch '3.2':
prevent writing to stderr from messing up the exception state (closes #14474)
http://hg.python.org/cpython/rev/60ad83716733 

Could the
 while thread._count() > c:
 pass
in test_thread.py be changed to this? (as used in other places)
 while thread._count() > c:
 time.sleep(0.01)
It currently hangs in Cython because it doesn't free the GIL during the plain C loop. (While that might be considered a bug in Cython, it's not what this test is supposed to test for.)