C and Python - different behaviour of the modulo (%) operation

1. Both variants are correct, however in mathematics (number theory in particular), Python's modulo is most commonly used.
2. In C, you do ((n % M) + M) % M to get the same result as in Python. E. g. ((-1 % 10) + 10) % 10. Note, how it still works for positive integers: ((17 % 10) + 10) % 10 == 17 % 10, as well as for both variants of C implementations (positive or negative remainder).

Python has a "true" modulo operation, while C has a remainder operation.

It has a direct relation with how the negative integer division is handled, i.e. rounded towards 0 or minus infinite. Python rounds towards minus infinite and C(99) towards 0, but in both languages (n/m)*m + n%m == n, so the % operator must compensate in the correct direction.

Ada is more explicit and has both, as mod and rem.

In C89/90 the behavior of division operator and remainder operator with negative operands is implementation-defined, meaning that depending on the implementation you can get either behavior. It is just required that the operators agree with each other: from a / b = q and a % b = r follows a = b * q + r. Use static asserts in your code to check the behavior, if it relies critically on the result.

In C99 the behavior you observe has become standard.

In fact, either behaviors have certain logic in it. The Python's behavior implements the true modulo operation. The behavior you observed is C is consistent with rounding towards 0 (it's also Fortran behavior).

One of the reasons the rounding towards 0 is preferred in C is that it is rather natural to expect the result of -a / b be the same as -(a / b). In case of true modulo behavior, -1 % 10 would evaluate to 9, meaning that -1 / 10 has to be -1. This might be seen as rather unnatural, since -(1 / 10) is 0.