Right-Side and In-Place Addition: __radd__ and __iadd__

Technically, the __add__ method that appeared in the prior example does not support the use of instance objects on the right side of the + operator. To implement such expressions, and hence support commutative-style operators, code the __radd__ method as well. Python calls __radd__ only when the object on the right side of the + is your class instance, but the object on the left is not an instance of your class. The __add__ method for the object on the left is called instead in all other cases:

>>> class Commuter:
...     def __init__(self, val):
...         self.val = val
...     def __add__(self, other):
...         print('add', self.val, other)
...         return self.val + other
...     def __radd__(self, other):
...         print('radd', self.val, other)
...         return other + self.val
...
>>> x = Commuter(88)
>>> y = Commuter(99)

>>> x + 1                      # __add__: instance + noninstance
add 88 1
89
>>> 1 + y                      # __radd__: noninstance + instance
radd 99 1
100
>>> x + y                      # __add__: instance + instance, triggers __radd__
add 88 <__main__.Commuter object at 0x02630910>
radd 99 88
187