Python __ifloordiv__() Magic Method

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Syntax

object.__ifloordiv__(self, other)

The Python __ifloordiv__() magic method implements the in-place floor division operation x //= y that calculates the integer 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.__ifloordiv__(y).
  • If this is not implemented, it tries the normal floor division operation x.__floordiv__(y).
  • If this is not implemented either, it tries reverse floor division operation y.__rfloordiv__(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 __ifloordiv__

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

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


x = Data()
y = Data()

x //= y

print(x)
# finxter 42

In-Place Floor Division //= Without __ifloordiv__()

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

Here’s an example:

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


x = Data()
y = Data()

x //= y

print(x)
# finxter 42

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

In-Place Floor Division //= Without __ifloordiv__() and __floordiv__()

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

class Data_1:
    pass

class Data_2:
    def __rfloordiv__(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 division x //= y but neither x.__ifloordiv__(y), nor x.__floordiv__(y), nor y.__rfloordiv(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'

Background Floor Division

The double-backslash // operator performs integer division and the single-backslash / operator performs float division. An example for integer division is 40//11 = 3. An example for float division is 40/11 = 3.6363636363636362.

>>> # Python 3
>>> 40//11
3
>>> 40/11
3.6363636363636362

You can learn more about Python division and integer division in this detailed blog guides:

Related Video

References:

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