Python __imod__() Magic Method

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Syntax

object.__imod__(self, other)

The Python __imod__() magic method implements the in-place modulo operation x %= y that calculates the modulo operation x % y, and assigns the result to the first operands variable x. This type of in-place 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.__imod__(y).
  • If this is not implemented, it tries the normal modulo operation x.__mod__(y).
  • If this is not implemented either, it tries reverse modulo operation y.__rmod__(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 __imod__

In the following code example, you create a class Data and define the magic method __imod__(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 modulo operation.

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


x = Data()
y = Data()

x %= y

print(x)
# finxter 42

In-Place Modulo %= Without __imod__()

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

Here’s an example:

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


x = Data()
y = Data()

x %= y

print(x)
# finxter 42

Even though the __imod__() method is not defined, the in-place modulo operation x %= y still works due to the __mod__() “fallback” magic method!

In-Place Modulo %= Without __imod__() and __mod__()

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

class Data_1:
    pass

class Data_2:
    def __rmod__(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 modulo x %= y but neither x.__imod__(y), nor x.__mod__(y), nor y.__rmod(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 Modulo

In Python like in many other programming languages, the modulo operator is represented by the percent % symbol. It returns the remainder of dividing the left by the right operand.

For example, the modulo operation 7%2 returns 1 because seven divided by two is three with remainder 1.

Python Modulo Explained (By Example)

The figure shows how the modulo operator works in Python for the example 7%3.

  • There’s a set with seven elements. This is the dividend of the operation.
  • The divisor is 3. You divide the set of seven elements into packages of three elements.
  • The remainder is 1. When dividing the set of seven elements into packages of three elements, one remains. This is the result of the modulo operation.

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

Related Video

References:

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