+= operator in python for __setitem__ (addition in place for square brackets)

Question

To define a class behaviour in the following statement:

my_object[item] = ...

I know I need to define the __setitem__ method.

What method do I need to define for the following statement:

my_object[item] += ...

Answer 1

my_object needs __getitem__ to retrieve the initial value of my_object[item] and __setitem__ to set the new value.

Additionally, Python needs a way to perform the addition. Either my_object[item] needs to implement the addition with __add__ or __iadd__, or the object on the right side of the += needs to implement __radd__.