2009/3/29 Guido van Rossum <report@bugs.python.org>:
>
> Guido van Rossum <guido@python.org> added the comment:
>
> I would love to use the time machine to kill the 'fail' variants as well
> as 'assert_'.  However they are in use and we don't want to break
> people's tests.  Breaking tests is painful.  And there are a lots of
> tests.  So we'll have to support them for a long, long time.  Maybe we
> can stop documenting them in 3.1 (this would also require changing
> things so that the 'def' lines all use the 'assert' variants).  Then
> maybe we can start deprecating them in 3.2 and 3.3, and perhaps remove
> them in 3.4.  I recommend we let 2.x alone, but 2to3 should fix all these.

Yes, actually this would be a great secondary use of 2to3's
infrastructure, and I think it could be done quite correctly because
people usually don't have many custom methods named failUnlessEqual()
etc.

Besides, they're tests, so people would know if they're broken. :)