Why does integer division truncate only for positive numerators in python?

Question

Why does integer division truncate when N // D --> N > 0 and N < D when N is positive, but doesn't when N is negative?

Example 12 // 25 = 0

BUT -12 // 25 = 1

This is in python 3.

It's not truncating; it's flooring. (The operator is called "floor division" because it always floors, even for floats.) — user2357112, Sep 27 '17 at 06:11

score 0 · Accepted Answer · answered Sep 27 '17 at 06:13

Sure, the answer here is that Python's integer division floors - it rounds down, always. So an instructive example would be something that doesn't yield 0, so it's more obvious:

>>> 10 // 3
3
>>> -10 // 3
-4

10/3 = 3.3333..., which rounds down to 3.
-10/3 = -3.3333...., which rounds down to -4.

So, even if it's a positive fractional part, like 7 // 4, which yields 1.75, Python rounds down to 1. The mathy explanation is in the linked blog post, this is just a mechanics explanation.

Why does integer division truncate only for positive numerators in python?

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