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The unittest module has some lovely enhanced comparison operations, but they're currently hard to access outside a test run.
It would be rather convenient if they were split out into a separate base class so they could be used directly without needing to create an actual test case.
Possible API:
 class Assertions:
 # All the enhanced assertion machinery goes here
 # Nothing related to actually running test cases
 class TestCase(Assertions):
 # This adds support for setUp, tearDown, addCleanup etc, etc
My current workaround is to just define a dummy "runTest" method in a subclass and use that:
>>> class Assertions(unittest.TestCase):
... def runTest(self): pass
... 
>>> check = Assertions()
>>> check.assertEqual([], ())
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib64/python2.7/unittest/case.py", line 511, in assertEqual
 assertion_func(first, second, msg=msg)
 File "/usr/lib64/python2.7/unittest/case.py", line 504, in _baseAssertEqual
 raise self.failureException(msg)
AssertionError: [] != ()
>>> check.assertEqual([1], [2])
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/usr/lib64/python2.7/unittest/case.py", line 511, in assertEqual
 assertion_func(first, second, msg=msg)
 File "/usr/lib64/python2.7/unittest/case.py", line 740, in assertListEqual
 self.assertSequenceEqual(list1, list2, msg, seq_type=list)
 File "/usr/lib64/python2.7/unittest/case.py", line 722, in assertSequenceEqual
 self.fail(msg)
 File "/usr/lib64/python2.7/unittest/case.py", line 408, in fail
 raise self.failureException(msg)
AssertionError: Lists differ: [1] != [2]
First differing element 0:
1
2
- [1]
+ [2]

Why not just instantiate a TestCase instance and use that?
>>> from unittest import TestCase
>>> t = TestCase()
>>> t.assertEqual('foo', 'bar')
Traceback (most recent call last):
 File "<stdin>", line 1, in <module>
 File "/compile/py3k-cpython/Lib/unittest/case.py", line 642, in assertEqual
 assertion_func(first, second, msg=msg)
 File "/compile/py3k-cpython/Lib/unittest/case.py", line 1021, in assertMultiLineEqual
 self.fail(self._formatMessage(msg, standardMsg))
 File "/compile/py3k-cpython/Lib/unittest/case.py", line 509, in fail
 raise self.failureException(msg)
AssertionError: 'foo' != 'bar'
- foo
+ bar

Because that doesn't work in 2.x (it fails with a ValueError due to "runTest" being missing) and I forgot you had already changed that behaviour to make it more convenient in 3.x :)