Python 2.7 documentation is VERY misleading about the functionality of assertItemsEqual. It actually claims to compare not only the counts, but the actual sorted elements themselves. This documentation mislead my group to use this method for comparing the elements. See https://hg.python.org/cpython/file/d9921cb6e3cd/Doc/library/unittest.rst:

   .. method:: assertItemsEqual(actual, expected, msg=None)

      Test that sequence *expected* contains the same elements as *actual*,
      regardless of their order. When they don't, an error message listing the
      differences between the sequences will be generated.

      Duplicate elements are *not* ignored when comparing *actual* and
      *expected*. It verifies if each element has the same count in both
      sequences. It is the equivalent of ``assertEqual(sorted(expected),
      sorted(actual))`` but it works with sequences of unhashable objects as
      well.