The problem with understanding augmented assignment is that it directs the compiler and interpreter to do one or maybe two mostly invisible optimizations. To me, the effective meaning of 'evalutating once versus twice' is most easily seen in the byte code generated by what is, remember, the reference implementation. What it does is what the less-than-super-clear doc means.
I posted the difference for one expression after looking at three other pairs. It is easy to examine such pairs in IDLE and, I presume, in other IDEs. Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
