l[0] += [1] is the same as l[0] = l[0] + [1] Does that make the reason for the error clearer? The problem is the attempt to assign a value to l[0]. It is not the same as e = l[0] e += [1] which is the equivalent to e = l[0] e = e + [1] This never assigns a value to l[0]. Schiavo Simon