> No "self mutation" as you suggest.

Your example would be the implementation for immutable Vector2Ds, and is implemented for you if you just wrote __add__. (I presume object.__iadd__ falls back to self.__add__.)

The only time you should actually override __iadd__ is for mutable objects, in which case it should look more like

  class Vector2D:
      def __iadd__(self, other)
          for i, x in enumerate(other):
              self[i] += x
          return self

That's the self mutation I was talking about.

Well, strictly that's not OK either, since operators shouldn't be duck-typed. Instead you should do

  class Vector2D:
      def __iadd__(self, other)
          if not isinstance(other, Vector2D):
              return NotImplemented

          for i, x in enumerate(other):
              self[i] += x
          return self