Index: operator.rst
===================================================================
--- operator.rst	(revision 84368)
+++ operator.rst	(working copy)
@@ -234,6 +234,23 @@
 ``z = operator.iadd(x, y)`` is equivalent to the compound statement
 ``z = x; z += y``.
 
+For immutable objects, (such as integers), ``a.__iadd__(b)`` returns
+``a + b`` *and then* this value is assigned to *a* (or rather *a*
+is bound to the value).  So for immutables objects, 
+``operator.iadd(a, b)`` is the same as ``a + b``.
+
+For mutable objects (such as lists), ``a.__iadd__(b)`` mutates the
+object *and then* returns self so that when the assignement is
+executed, *a* will still be bound the the same object.  E.g. if 
+``a = [1, 2]`` then::
+
+    a += [3]
+
+will first append 3 to the list and then reassign the list to
+*a* (it is unnecessary in this case but if this step was omitted,
+the "in place" operators wouldn't work on immutables types).
+
+
 .. function:: iadd(a, b)
               __iadd__(a, b)