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When calling assertItemsEqual(first,second), if the items in a and b differ, the AssertionError treats second as the first sequence and first as the second sequence.
Repro code:
>>> from unittest.case import TestCase
>>> class Foo(TestCase):
... def runTest(self):
... self.assertItemsEqual([1],[])
... 
>>> Foo().runTest()
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "<stdin>", line 3, in runTest
 File "/usr/lib/python2.7/unittest/case.py", line 899, in assertItemsEqual
 self.fail(msg)
 File "/usr/lib/python2.7/unittest/case.py", line 408, in fail
 raise self.failureException(msg)
AssertionError: Element counts were not equal:
First has 0, Second has 1: 1
This happens because of this code in unittest/case.py:
def assertItemsEqual(self, expected_seq, actual_seq, msg=None):
 # (skip docstring)
 first_seq, second_seq = list(actual_seq), list(expected_seq)
list(actual_seq) is assigned to first_seq, even though it's actually the second argument.
This would be fixed either by swapping expected_seq and actual_seq in the function's arguments, or swapping the assignment of first_seq and second_seq.

Duplicate of 14832.