In the unittest module, TestCase class:

If one wants to provide a more descriptive fail message compared to the
default, it is often valuable to be able to refer to the arguments
evaluated by the fail*- (or assert*-) method. This can be accomplished
by binding names to the evaluated values before the call to the
fail*-methods, and then providing a string with the values embedded as
needed. This, however, can be quite cumbersome when there are a lot of
calls to fail*-methods.

Today:
Arg1 = RealValue()
Arg2 = ExpectedValue()
self.failUnlessEqual(Arg1, Arg2, "Got {0}, but expected
{1}.".format(Arg1, Arg2))

Proposed solution:
In the fail*-methods, check if the msg argument is some kind of string
(basestring?), and run the format() method on the string with the
supplied arguments. This would result in code like this:

self.failUnlessEqual(RealValue(), ExpectedValue(), "Got {0}, but
expected {1}.")