Python __mod__() Magic Method

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Syntax

object.__mod__(self, other)

The Python __mod__() method implements the modulo operation % that per default returns the remainder of dividing the left by the right operand. Internally, Python attempts to call x.__mod__(y) to implement the modulo operation x%y. If the method is not implemented, Python first 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.

Example

In the following example, you create a custom class Data and overwrite the __mod__() method so that it returns a dummy string when trying to calculate the modulo of two numbers.

class Data:
        
    def __mod__(self, other):
        return '... my result of mod...'


a = Data()
b = Data()
c = a % b

print(c)
# ... my result of mod...

If you hadn’t defined the __mod__() method, Python would’ve raised a TypeError.

How to Resolve TypeError: unsupported operand type(s) for %

Consider the following code snippet where you try to calculate the modulo of two custom objects without defining the dunder method __mod__():

class Data:
    pass


a = Data()
b = Data()
c = a % b

print(c)

Running this leads to the following error message on my computer:

Traceback (most recent call last):
  File "C:\Users\xcent\Desktop\code.py", line 7, in <module>
    c = a % b
TypeError: unsupported operand type(s) for %: 'Data' and 'Data'

The reason for this error is that the __mod__() dunder method has never been defined—and it is not defined for a custom object by default. So, to resolve the TypeError: unsupported operand type(s) for %, you need to provide the __mod__(self, other) method in your class definition as shown previously:

class Data:
        
    def __mod__(self, other):
        return '... my result of mod...'

Python __mod__ vs __rmod__

Say, you want to calculate the modulo of two custom 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 operand 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), the result would be wrong because the modulo operation is non-commutative. That’s why y.__rmod__(x) is needed.

So, 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 modulo method defined on object x.

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

class Data_1:
    pass

class Data_2:
    def __rmod__(self, other):
        return 'called rmod'


x = Data_1()
y = Data_2()

print(x % y)
# called rmod

References:

Explainer Video Modulo

You can also check out my explainer video where I’ll give you a deep dive on the built-in modulo operation and how to use them for various data types. Click to watch:

Where to Go From Here?

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