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

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@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

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