Using unittest.TestCase.longMessage to provide more detail. See the example below and output. Documentation: https://docs.python.org/2/library/unittest.html#unittest.TestCase.longMessage

long_message_tests.py

""" Demo the use of longMessage to give more details """

import unittest


class MyTestCase(unittest.TestCase):
    def test_use_long_message(self):
        """ Use longMessage for more detailed error message """
        self.longMessage = True
        actual_values = [0, 1, 2, 3]
        expected_values = [0, 1, 2, 3, 4]
        self.assertEqual(expected_values, actual_values, "Items mismatched")

    def test_not_use_long_message(self):
        """ Not using longMessage, error will be terse """
        self.longMessage = False
        actual_values = [0, 1, 2, 3]
        expected_values = [0, 1, 2, 3, 4]
        self.assertEqual(expected_values, actual_values, "Items mismatched")


if __name__ == '__main__':
    unittest.main()

output.txt

FF
======================================================================
FAIL: test_not_use_long_message (long_message_tests.MyTestCase)
Not using longMessage, error will be terse
----------------------------------------------------------------------
Traceback (most recent call last):
  File "long_message_tests.py", line 19, in test_not_use_long_message
    self.assertEqual(expected_values, actual_values, "Items mismatched")
AssertionError: Items mismatched

======================================================================
FAIL: test_use_long_message (long_message_tests.MyTestCase)
Use longMessage for more detailed error message
----------------------------------------------------------------------
Traceback (most recent call last):
  File "long_message_tests.py", line 12, in test_use_long_message
    self.assertEqual(expected_values, actual_values, "Items mismatched")
AssertionError: Lists differ: [0, 1, 2, 3, 4] != [0, 1, 2, 3]

First list contains 1 additional elements.
First extra element 4:
4

- [0, 1, 2, 3, 4]
?            ---

+ [0, 1, 2, 3] : Items mismatched

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

FAILED (failures=2)

1. The longMessage shows more detailed output, how the two lists differ
2. The use of descriptive names such as expected_values and actual_values helps distinguishing between the two. By looking at the output, we know which list contains the expected values and actual values.
3. Notice the use of The first line of docstring in the output