python - 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:

>>> i=42
>>> id(i)
10021708
>>> i.__iadd__(1)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'int' object has no attribute '__iadd__'
>>> i += 1
>>> i
43
>>> id(i)
10021696

Not surprisingly, int objects have no __iadd__() method, yet applying += to one doesn't result in an error, instead it apparently creates a new int and also somehow magically reassigns it to the name given in the augmented assignment statement.

Is it possible to create a user-defined class or subclass of a built-in immutable one that does this, and if so, how?

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.