If a function passed to .addOnException itself raises an exception, the test is treated as passing.

[tomprince]

from testtools import TestCase

class Tests(TestCase):
    def test_thing(self):
        def raiser(_):
            raise Exception("cascade")
        self.addOnException(raiser)
        self.fail()

gives

Tests running...

Ran 1 test in 0.001s
OK
Traceback (most recent call last):
  File "/usr/lib64/python2.7/runpy.py", line 174, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib64/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "../lib/python2.7/site-packages/testtools/run.py", line 267, in <module>
    main(sys.argv, sys.stdout)
  File "../lib/python2.7/site-packages/testtools/run.py", line 264, in main
    stdout=stdout)
  File "../lib/python2.7/site-packages/testtools/run.py", line 187, in __init__
    self.runTests()
  File "../lib/python2.7/site-packages/testtools/run.py", line 221, in runTests
    self.result = testRunner.run(self.test)
  File "../lib/python2.7/site-packages/testtools/run.py", line 107, in run
    return test.run(result)
  File "../lib/python2.7/site-packages/unittest2/suite.py", line 126, in run
    test(result)
  File "../lib/python2.7/site-packages/unittest2/suite.py", line 87, in __call__
    return self.run(*args, **kwds)
  File "../lib/python2.7/site-packages/unittest2/suite.py", line 126, in run
    test(result)
  File "../lib/python2.7/site-packages/unittest2/suite.py", line 87, in __call__
    return self.run(*args, **kwds)
  File "../lib/python2.7/site-packages/unittest2/suite.py", line 126, in run
    test(result)
  File "../lib/python2.7/site-packages/unittest2/case.py", line 673, in __call__
    return self.run(*args, **kwds)
  File "../lib/python2.7/site-packages/testtools/testcase.py", line 675, in run
    return run_test.run(result)
  File "../lib/python2.7/site-packages/testtools/runtest.py", line 80, in run
    return self._run_one(actual_result)
  File "../lib/python2.7/site-packages/testtools/runtest.py", line 94, in _run_one
    return self._run_prepared_result(ExtendedToOriginalDecorator(result))
  File "../lib/python2.7/site-packages/testtools/runtest.py", line 108, in _run_prepared_result
    self._run_core()
  File "../lib/python2.7/site-packages/testtools/runtest.py", line 144, in _run_core
    self.case._run_test_method, self.result):
  File "../lib/python2.7/site-packages/testtools/runtest.py", line 193, in _run_user
    return self._got_user_exception(sys.exc_info())
  File "../lib/python2.7/site-packages/testtools/runtest.py", line 213, in _got_user_exception
    self.case.onException(exc_info, tb_label=tb_label)
  File "../lib/python2.7/site-packages/testtools/testcase.py", line 626, in onException
    handler(exc_info)
  File "test_thing.py", line 6, in raiser
    raise Exception("cascade")
Exception: cascade

When using AsynchronousDeferredRunTest:

from testtools import TestCase                      
from testtools.twistedsupport import (              
    AsynchronousDeferredRunTest,                    
)                                                   
                                                    
class Tests(TestCase):                              
    run_tests_with = AsynchronousDeferredRunTest    
    def test_thing(self):                           
        def raiser(_):                              
            raise Exception("cascade")              
        self.addOnException(raiser)                 
        self.fail()                                 

the failure is entirely swallowed:

Tests running...

Ran 1 test in 0.063s
OK