Here is a useful trick to restore the normal call syntax for delayed
evaluation for assertRaises():

     from contextlib import contextmanager

     @contextmanager
     def raised(exctype):
         try:
             yield
             raise AssertionError("Exception %s not raised." %
exctype.__name__)
         except exctype, e:
             pass

Then you can do this::

    with raised(ValueError):
        rate = foo(arg1, arg2)

Instead of ::

    self.assertRaises(foo, arg1, arg2)

Which means you don't have to break the function from its arguments (it
doesn't look like a function call anymore).

P.S. I know I didn't include self up there in my proposed implementation
but it could be parameterized to get the failureException. 

(This is a second issue:)
However, I really think that all the failUnlessXXX methods should be
taken out of the class and made available at module level so that they
can be reused by py.test tests as well, and they are perhaps even worthy
of their own module. I can't remember a single instance where I had to
override that failureException class...

Comments welcome, I'd be happy to fix this one myself if we can restore
my commit privileges (I think they expired because I didn't use them
since the need-for-speed workshop.)

This is already done in trunk, you can use it as follows:

with self.assertRaises(ZeroDivisionError):
    1/0

Humph!  The time machine strikes again.