> My intuitions come from C and C++, so "+=" should always modify a single object (in C lingo it would be "lvalue") in place. Storing the result somewhere else than the left side of "+=" is counterintuitive to me.

I thought of this as though the list A.b was marked static - then any modification of b through any instance of A modifies the shared static member

  >>> class A:
  ...   b = [1,]
  ...
  >>> obj = A()
  >>> obj.b.append(2)
  >>> A.b
  [1, 2]
  >>> foo = A()
  >>> foo.b
  [1, 2]
  >>>