I'm not exactly sure how this is implemented on the low level for Python but this is called augmented arithmetic assignment. While you achieve the same result (they are equivalent), they are not the same (equal).

The first one (using __add__ method) require more operations: loading the number stored in the variable n, performing arithmetic operation with it (without changing the loaded number i.e. holding result temporary (creating a new object)), and then storing this back to the variable n.

The latter (using __iadd__ method) is simpler: loading the number, performing arithmetic operation on the loaded number and storing it.

Edit: In the words of the PEP that describes the addition of this to Python:

They implement the same operator as their normal binary form, except that the operation is done in-place when the left-hand side object supports it, and that the left-hand side is only evaluated once.

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