On 3 Feb., 11:47, John O'Hagan <resea...@johnohagan.com> wrote: > But isn't it equally true if we say that z = t[1], then t[1] += x is > syntactic sugar for z = z.__iadd__(x)? Why should that fail, if z can handle > it?
It's more like syntactic sugar for y = t; z = y.__getitem__(1); z.__iadd__(x); y.__setitem__(1, z) It's clear that only the last expression fails, after the mutation has taken place. Just in case you wonder about the y: you need it for more complicated cases. t[1][1] += [4] is syntactic sugar for y = t.__getitem__(1); z = y.__getitem__(1); z.__iadd__([4]); y.__setitem__(1, z) That makes clear why there's no exception in this code: >>> t = (0, [1, [2, 3]]) >>> t[1][1] += [4] >>> t (0, [1, [2, 3, 4]]) -- http://mail.python.org/mailman/listinfo/python-list
