How to implement “__iadd__()” for an immutable type?

Question

I would like to subclass an immutable type or implement one of my own which behaves like an int does as shown in the following console session:

Answer 1

Simply don't implement __iadd__, but only __add__:

>>> class X(object):
...     def __add__(self, o):
...             return "added"
>>> x = X()
>>> x += 2
>>> x
'added'

If there's no x.__iadd__, Python simply calculates x += y as x = x + y doc.