Python __rmod__() Magic Method

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object.__rmod__(self, other)

The Python __rmod__() method implements the reverse modulo operation with reflected, swapped operands. So, when you call x % y, Python attempts to call x.__mod__(y). If the method is not implemented, Python attempts to call __rmod__ on the right operand and if this isn’t implemented either, it 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.

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.

>>> 7 % 2

To understand this operation in detail, feel free to read over our tutorial or watch the following video:

Python Modulo - A Simple Illustrated Guide

What’s the Difference Between __mod__() and __rmod__()?

Say, you want to use the percentage operator on two objects x and y:

print(x % y)

Python first tries to call the left object’s __mod__() method x.__mod__(y). But this may fail for two reasons:

  1. The method x.__mod__() is not implemented in the first place, or
  2. The method x.__mod__() is implemented but returns a NotImplemented value indicating that the data types are incompatible.

If this fails, Python tries to fix it by calling the y.__rmod__() for reverse modulo on the right operator y.

If this method is implemented, Python knows that it doesn’t run into a potential problem of a non-commutative operation. If it would just execute y.__mod__(x) instead of x.__mod__(y), it could cause an error if the operation is non-commutative. That’s why y.__rmod__(x) is needed.

The difference between x.__mod__(y) and x.__rmod__(y) is that the former calculates x % y whereas the latter calculates y % x — both calling the respective true division method defined on object x.

You can see this in effect here where we attempt to call the operation on the left operand x—but as it’s not implemented, Python simply calls the reverse operation on the right operand y.

class Data_1:

class Data_2:
    def __rmod__(self, other):
        return 'called reverse % operator'

x = Data_1()
y = Data_2()

print(x % y)
# called reverse % operator


Where to Go From Here?

Enough theory. Let’s get some practice!

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