I think {,} is intuitive because it reminds me of syntax for one-element tuples, e.g. (1,) -- but it may make code harder to read, which is not Pythonic.

I didn't think to do {,} which I like. s{} looks weird to me, but I couldn't tell you why.

The s{} alternatve is based on the strange r'' or u'' syntax i've gotten used to.