One possible argument for making PASS the default, even if that means implementation-dependent behaviour with NANs, is that in the absense of a clear preference for FAIL or RETURN, at least PASS is backwards compatible.

You might shoot yourself in the foot, but at least you know its the same foot you shot yourself in using the previous version *wink* 

I've lost attribution chain. I think this is Steven, but it doesn't really matter.

This statement is untrue, or at least only accidentally true at most. The behavior of sorted() against partially ordered collections is unspecified. The author of Timsort says exactly this.

If stastics.median() keeps the same implementation—or keeps it with a PASS argument—it may or may not produce the same result in a later Python versions. Timsort is great, but even that has been tweaked sightly over time.

I guess the statement is true if "same foot" means "meaningless answer" not some specific value. But that hardly feels like a defense of the behavior.