> a case where x+=y was not equivalent to x=x+y This is true for lists in Python. `a = a + b` creates a new list, but `a += b` works in-place: >>> a = [1,

> a case where x+=y was not equivalent to x=x+y This is true for lists in Python. `a = a + b` creates a new list, but `a += b` works in-place: >>> a = [1, 2] >>> b = a >>> a = a + [3] >>> print(a, b) [1, 2, 3] [1, 2] >>> a = [1, 2] >>> b = a >>> a += [3] >>> print(a, b) [1, 2, 3] [1, 2, 3] There are other issues with the `+=`-style operators as well - IMO it was a mistake to have `a += b` be anything other than a shorthand for `a = a + b`.