Actually, in Python 2.6, both assertEqual and assertEquals are convenience aliases to failUnlessEqual. The source declares them thus:

 # Synonyms for assertion methods assertEqual = assertEquals = failUnlessEqual

In Python 3, to your point, failUnlessEqual is explicitly deprecated. assertEquals carries this comment :-)

Synonyms for assertion methods

The plurals are undocumented. Keep them that way to discourage use.

Do not add more. Do not remove.

Going through a deprecation cycle on these would annoy many people.

So, the upshot appears to be that you should use whatever you like for Python 2.x, but tend toward assertEqual for Python 3.

A 3.3 update: From 26.3.7.1.1. Deprecated aliases :

For historical reasons, some of the TestCase methods had one or more aliases that are now deprecated. The following table lists the correct names along with their deprecated aliases:

Method Name   | Deprecated alias | Deprecated alias--------------+------------------+-----------------assertEqual() | failUnlessEqual  | assertEquals...

Not just for Python 3.x, since Python 2.7 assertEquals has been deprecated as well:

Method Name            | Deprecated alias(es)_________________________________________________________assertEqual()          | failUnlessEqual, assertEquals

From 25.3.7.1.1. Deprecated aliases