Python __itruediv__() Magic Method

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object.__itruediv__(self, other)

The Python __itruediv__() magic method implements the in-place division operation x /= y that calculates the division operation x / y, and assigns the result to the first operands variable x. 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.__itruediv__(y).
  • If this is not implemented, it tries the normal true division operation x.__truediv__(y).
  • If this is not implemented either, it tries reverse true division operation y.__rtruediv__(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.

Note that this is different from the integer division operator x // y. You can learn about integer division in this Finxter blog tutorial.

Basic Example Overriding __itruediv__

In the following code example, you create a class Data and define the magic method __itruediv__(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 true division operation.

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

x = Data()
y = Data()

x /= y

# finxter 42

In-Place True Division /= Without __itruediv__()

To support in-place true division on a custom class, you don’t have to overwrite the in-place __itruediv__() method. Because if the method is not defined, Python will fall back to the normal __truediv__() method and assign its result to the first operand.

Here’s an example:

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

x = Data()
y = Data()

x /= y

# finxter 42

Even though the __itruediv__() method is not defined, the in-place true division operation x /= y still works due to the __truediv__() “fallback” magic method!

In-Place True Division /= Without __itruediv__() and __truediv__()

To support in-place true division x /= y on a custom class, you don’t even have to overwrite any of the x.__itruediv__(y) or x.__truediv__(y) methods. If both are not defined, Python falls back to the reverse y.__rtruediv__(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 true division operation. Then you define a custom class for the second operand that defines the __rtruediv__() method. For the in-place operation, Python falls back to the __rtruediv__() method defined on the second operand and assigns it to the first operand x:

class Data_1:

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

x = Data_1()
y = Data_2()

x /= y

# finxter 42

TypeError: unsupported operand type(s) for /=

If you try to perform in-place division x /= y but neither x.__itruediv__(y), nor x.__truediv__(y), nor y.__rtruediv(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:

x = Data()
y = Data()

x /= y


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

Background True Division

In Python 3, the single front-slash “/” is a float division operator that returns a float value as a result. For example, the expression 10/4 returns 2.5 instead of 2, and 4/2 returns 2.0 instead of 2.

>>> # Python 3
>>> 10/4
>>> 4/2

Be careful to use the most updated type of Python available. For example, Python 2.0 returns an integer instead of a float value for the / operator. Also when we perform division in Python we want to be careful what value we divide by. We find that if we divide by a whole number, it will round to an integer.

>>> 10 / 90

You can find full tutorials on related operators (including videos) here:

Related Video

[Overview] Python's In-Place Assignment Operators | Compound Operators


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