The example look ill-conceived to begin with. If you want to extend the list that is the first element of the tuple just use t[0].extend([1]). Using an augmented assignment operator makes no sense if the left hand side can not be assigned to. I don't know Python well enough to tell why it does not check if the left hand side can be assigned to before evaluating the right hand side, i.e. why it dos not check that __setitem__ is present. This would make the behavior more similar to a compiled statically typed language where this would be a compile time error and the right hand side would never get evaluated.