Division by 1: just when you thought you knew Python

To the hard people that came from ruthless land of C, many things in Python look like craziness. But with the years are passing, you get to feel like at home with it… until you see something like x % 1.

Why would anyone do that? In production code?

The Docs ¶

The Python documentation gives a bit of clue regarding the possible uses of such expression:

The arguments may be floating point numbers, e.g., 3.14 % 0.7 equals 0.34 (since 3.14 equals 4 * 0.7 + 0.34.)

And later on:

the absolute value of the result is strictly smaller than the absolute value of the second operand.

So, contrary to my C-fostered intuition, the dividend does not have to be integer. The modulo operator for x % y in this case returns such number that x = r * y + (x % y) for the largest possible integer r (well, for the negative numbers it’s a bit more complicated, but the idea is the same).

This means that when you take modulo by 1 what you get is… right, the fractional part of x. And in a much more concise way then usual math.modf.

The Examples ¶

Let’s try this:

>>> x = 3.14
>>> x % .7
0.3400000000000003
>>> x % 1
0.14000000000000012

As you see, this works almost as in docs. Well, that’s floating point arithmetic for you =).

The End ¶

It worth pointing out that Python is not unique in this respect. While in C both operands of the % operator will be cast to int, in most interpreted languages (including Ruby and JS), and, quite surprisingly, in Java, % is defined for floating point numbers in pretty much the same way as it is in Python.

Well, the more you learn…