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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}.")

The new longMessage class attribute on TestCase already shows both
arguments when a call to assertEqual fails (the failUnless methods are
now deprecated) - even if you supply a custom message.

I admit that I had not seen the longMessage attribute. That is better
than the present possibilities in 2.6, although not as configurable as
my suggestion (but with better backwards compatibility).
Personally I will be a user of the longMessage feature, dreaming of even
more functionality.

Changing unittest.TestCase.failUnlessEqual() to something like this will be very useful:
 def failUnlessEqual(self, first, second, msg=None):
 """Fail if the two objects are unequal as determined by the '=='
 operator. Argument msg could optionally include string format
 operators named "lhs" and "rhs" (e.g., "%(lhs)r")
 """
 if not first == second:
 raise self.failureException, \
 (msg or "%(lhs)r != %(rhs)r") % dict(lhs=10, rhs=20)

Oops... the dict part should have been "dict(lhs=first, rhs=second)":
 def failUnlessEqual(self, first, second, msg=None):
 """Fail if the two objects are unequal as determined by the '=='
 operator. Argument msg could optionally include string format
 operators named "lhs" and "rhs" (e.g., "%(lhs)r")
 """
 if not first == second:
 raise self.failureException, \
 (msg or "%(lhs)r != %(rhs)r") % dict(lhs=first, rhs=second)

This appears to be me to be obsolete, given that long messages are now the default, and the message argument enhances the long message rather than replacing it.