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The idea of using difflib to contract the failure   
messages is a great one.  Having had some   
exposure to GUI front-ends for JUnit, such as those   
available in IDEA and Eclipse, it seems to me that all   
the information available at the point of failure should   
be stored, and only formatted for display by the   
TestRunner at a later point. Eclipse, for example, can  
show long expected and actual value panes in a  
colour-code side-by-side diff viewer, which is even  
better than the difflib output.  
  
Neither the current nor the patched version of unittest  
take the approach of delegating failure message  
formatting to the Runner, but I'd prefer to see things  
move in that direction that to merge this patch in its  
current form.  
  
I guess that a richer hierarchy of failure exceptions  
would be required, so that TestRunners could elect  
to treat specific failures in their own preferred way.