## Syntax

object.__imatmul__(self, other)

The Python `__imatmul__()`

magic method implements in-place matrix multiplication `x @= y`

that calculates the matrix multiplication of the two 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.__imatmul__(y)`

. - If this is not implemented, it tries the normal matrix multiplication
`x.__matmul__(y)`

. - If this is not implemented either, it tries reverse matrix multiplication
`y.__rmatmul__(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 __imatmul__

In the following code example, you create a class `Data`

and define the magic method `__imatmul__(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 matrix multiplication.

class Data: def __imatmul__(self, other): return 'finxter 42' x = Data() y = Data() x @= y print(x) # finxter 42

## In-Place Matrix Multiplication @ Without __imatmul__()

To support in-place matrix multiplication on a custom class, you don’t have to overwrite the in-place `__imatmul__()`

method. Because if the method is not defined, Python will fall back to the normal `__matmul__()`

method and assign its result to the first operand.

Here’s an example:

class Data: def __matmul__(self, other): return 'finxter 42' x = Data() y = Data() x @= y print(x) # finxter 42

Even though the `__imatmul__()`

method is not defined, the in-place matrix multiplication operation `x @= y`

still works due to the `__matmul__()`

“fallback” magic method!

## In-Place Matrix Multiplication Without __imatmul__() and __matmul__()

To support in-place multiplication `x @= y`

on a custom class, you don’t even have to overwrite any of the `x.__imatmul__(y)`

or `x.__matmul__(y)`

methods. If both are not defined, Python falls back to the reverse `y.__rmatmul__(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 the matrix multiplication operation. Then you define a custom class for the second operand that defines the `__rmatmul__()`

method. For the in-place operation, Python falls back to the `__rmatmul__()`

method defined on the second operand and assigns it to the first operand `x`

:

class Data_1: pass class Data_2: def __rmatmul__(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 multiplication `x @= y`

but neither `x.__imatmul__(y)`

, nor `x.__matmul__(y)`

, nor `y.__rmatmul(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:**

## Where to Go From Here?

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