WRT assuming the presence of the + optimization: I think it makes less sense to assume that % is faster. + is the concatenation operator, so it’s reasonable to assume that that’s the best way to concatenate; until 2.4, that wasn’t true, but now it is (for plain strings).

“Here’s the summary: counting on this optimization is unwise, as it turns out to depend on low-level details of system memory allocation that can vary from system to system and workload to workload. Also, this is only for plain (byte) strings, not for Unicode strings; as of Python 2.4.2, Unicode string concatenation remains un-optimized.”

Perhaps we could get a new set of benchmark timings with unicode strings?

+: 0.512433052063
str.join(list): 0.9431848526
str.join(tuple): 0.859102964401
%: 0.936084985733
–Alternate versions that don’t re-create the string every time–
+: 0.50304889679
str.join(list): 0.887317895889
str.join(tuple): 0.794829130173
%: 0.89204788208