http://docs.python.org/3/library/unittest.html#unittest.TestCase.longMessage "If set to True then any explicit failure message you pass in to the assert methods will be appended to the end of the normal failure message. The normal messages contain useful information about the objects involved, for example the message from assertEqual shows you the repr of the two unequal objects. Setting this attribute to True allows you to have a custom error message in addition to the normal one." the last sentence duplicates the first one
I think the text is OK as it stands. The first sentence is a technical description of the result of setting the attribute, the last sentence is a conceptual discussion of why you would want to set the attribute.
