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Message: [QUOTE="Wolfram Hinderer, post: 4686739"] 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: (0, [1, [2, 3, 4]]) [/QUOTE]