I think it heavily depends on what is "x". If x is bound to a mutable x=x+1 and x+=1 can not only have different speed but indeed can do two very unrelate things (the former probably binding to a new object, the latter probably modifying the same object). For example consider what happens with lists and [1] instead of 1...
>>> s = [] >>> t = s >>> t = t + [1] >>> t [1] >>> s [] >>> s2 = [] >>> t2 = s2 >>> t2 += [1] >>> t2 [1] >>> s2 [1] >>> Also if x is not a single name but a more convoluted expression with += that expression is evaluated once and even in this case there can be differences in speed and not only in speed. -- http://mail.python.org/mailman/listinfo/python-list
