I suspect that what Antoine is referring to is the fact that Python 3.1 has this behavior.  Whether or not it is explicitly documented is a secondary issue.

We're having a similar issue in the unittest package, where there's a new function, assertSameElements, that has an unfortunate and poorly documented API.  But changing that API now that the function exists in a released version (3.1) is not something to be done lightly, if it is done at all.

This is definitely an unfortunate state of affairs no matter how you look at it.