# Python __rmod__() Magic Method

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## Syntax

`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
1```

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

## 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:
pass

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

x = Data_1()
y = Data_2()

print(x % y)
# called reverse % operator
```

References:

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