Posted on: 2025-08-26

I was writing code in Python that required a modulo. The code worked until the number was negative.

-321 % 10

Applying a modulo 10 to a number is an easy way to get the last digit. 321 modulo 10 is 1. I was expecting -321 modulo 10 to be 1. That is the case in JavaScript, not in Python. In Python, -321 modulo 10 is 9.

Down to Infinity

In Python, the operator % follows this formula:

a % b == a - b * math.floor(a / b)

Using our formal example:

-321 % 10 == -321 - 10 * math.floor(-321/10)
          == -321 - 10 * math.floor(-32.1)
          == -321 - 10 * -33
          == -321 - -330
          == -321 + 330
          == 9

The reason is that the divisor (the number 10) is positive, thus the answer will lean toward a positive number instead of -1.

To get the same behavior as JavaScript, who lean toward zero instead of negative infinity in that case, you need to use the math library:

1 === math.remainder(-321, 10)

Conclusion

JavaScript's way is used by C++, Java, C#, Rust, GO and PHP. These are all languages I am familiar with, thus I assumed it was always acting that way.

Python's approach is shared with languages I have never coded with, such as Haskell, Erlang, and OCaml.