How can I hide my stack frames in a TestCase subclass?

[David Banks]

I want to add a custom assert method to a TestCase subclass.  I tried to
copy my implementation from the unittest module so that it would match
the behaviour of the regular TestCase as closely as possible.  (I would
prefer to just delegate to self.assertEqual() but this causes even more
backtrace noise, see below.)  The unittest module seems to automatically
hide some internal details of its implementation when reporting failed
assertions.

  import unittest

  class MyTestCase(unittest.TestCase):
      def assertLengthIsOne(self, sequence, msg=None):
          if len(sequence) != 1:
              msg = self._formatMessage(msg, "length is not one")
              raise self.failureException(msg)

  class TestFoo(MyTestCase):
      seq = (1, 2, 3, 4, 5)

      def test_stock_unittest_assertion(self):
          self.assertEqual(len(self.seq), 1)

      def test_custom_assertion(self):
          self.assertLengthIsOne(self.seq)


  unittest.main()

The output of this is as such:

  amoe at vuurvlieg $ python unittest-demo.py
  FF
  ======================================================================
  FAIL: test_custom_assertion (__main__.TestFoo)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "unittest-demo.py", line 16, in test_custom_assertion
      self.assertLengthIsOne(self.seq)
    File "unittest-demo.py", line 7, in assertLengthIsOne
      raise self.failureException(msg)
  AssertionError: length is not one

  ======================================================================
  FAIL: test_stock_unittest_assertion (__main__.TestFoo)
  ----------------------------------------------------------------------
  Traceback (most recent call last):
    File "unittest-demo.py", line 13, in test_stock_unittest_assertion
      self.assertEqual(len(self.seq), 1)
  AssertionError: 5 != 1

  ----------------------------------------------------------------------
  Ran 2 tests in 0.000s

  FAILED (failures=2)

Note that the custom assert method causes a stack trace with two frames,
one inside the method itself, whereas the stock unittest method only has
one frame, the relevant line in the user's code.  How can I apply this
frame-hiding behaviour to my own method?