Python __iadd__() Magic Method

Syntax

object.__iadd__(self, other)

The Python __iadd__() magic method implements in-place addition x += y that adds together the operands and assigns the result to the left operand. This operation is also called augmented arithmetic assignment. The method simply returns the new value to be assigned to the first operand.

  • When you call x += y, Python first attempts to call x.__iadd__(y).
  • If this is not implemented, it tries the normal addition x.__add__(y).
  • If this is not implemented either, it tries reverse addition y.__radd__(x) with swapped operands.

The result is then assigned to the first operand x. If none of those operations is implemented, Python raises a TypeError.

We call this a “Dunder Method” for Double Underscore Method” (also called “magic method”). To get a list of all dunder methods with explanation, check out our dunder cheat sheet article on this blog.

Basic Example Overriding __iadd__

In the following code example, you create a class Data and define the magic method __iadd__(self, other).

  • The “self” argument is the default argument of each method and it refers to the object on which it is called—in our case, the first operand of the in-place operation.
  • The “other” argument of the in-place method refers to the second operand, i.e., y in the in-place operation x += y.

The return value of the operation returns a dummy string 'finxter 42' to be assigned to the first operand. In practice, this would be the result of the in-place addition.

class Data:
    def __iadd__(self, other):
        return 'finxter 42'


x = Data()
y = Data()

x += y

print(x)
# finxter 42

In-Place Addition Without __iadd__()

To support in-place addition on a custom class, you don’t have to overwrite the in-place __iadd__() method. Because if the method is not defined, Python falls back to the normal __add__() method and assigns its result to the first operand.

Here’s an example:

class Data:
    def __add__(self, other):
        return 'finxter 42'


x = Data()
y = Data()

x += y

print(x)
# finxter 42

In-Place Addition Without __iadd__() and __add__()

To support in-place addition x += y on a custom class, you don’t even have to overwrite any of the x.__iadd__(y) or x.__add__(y) methods. If both are not defined, Python falls back to the reverse y.__radd__(x) method and assigns its result to the first operand.

Here’s an example where you create a custom class for the first operand that doesn’t support any addition operation. Then you define a custom class for the second operand that defines the __radd__() method. For the in-place operation, Python falls back to the __radd__() method defined on the second operand and assigns it to the first operand x:

class Data_1:
    pass

class Data_2:
    def __radd__(self, other):
        return 'finxter 42'

x = Data_1()
y = Data_2()

x += y

print(x)
# finxter 42

TypeError: unsupported operand type(s) for +=

If you try to perform in-place addition x += y but neither x.__iadd__(y), nor x.__add__(y), nor y.__radd(x) is defined, Python raises a “TypeError: unsupported operand type(s) for +=“. To fix this error, simply define any of those methods before performing the in-place operation.

class Data:
    pass


x = Data()
y = Data()

x += y

Output:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 8, in <module>
    x += y
TypeError: unsupported operand type(s) for +=: 'Data' and 'Data'

Related Video

References:

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