Today I learned how to do ceiling division in Python just with //.

Floor division //

I recently published a tweet telling people about the floor division operator in Python, //:

This operator is equivalent to doing regular division and then flooring down:

>>> # How many years in 10_000 days?
>>> from math import floor; floor(10_000 / 365)
>>> 10_000 // 365

Then, someone asked if Python also had a built-in for ceiling division, that is, an operator that divided the operands and then rounded up.

While there is no direct built-in operator for that, someone replied saying that we can use a couple of minus signs and floor division to do that.

Ceiling division with a and b would be equivalent to ceil(a / b). And they showed that we can do it with -(-a // b):

>>> from math import ceil
>>> a, b = 10, 3
>>> ceil(a / b)
>>> -(-a // b)

Why does this work?

floor rounds down and ceil rounds up. By using -a in the division, it's as if you flip a upside down, so “its ceiling is now on the floor”, so you can use -a // b. Then, you just need to put everything back in place, using a final negation: -(-a // b).

At first, I thought this would fail for some combination of positive/negative values for a and b, but it most certainly doesn't.

For one, the explanation works regardless of the sign of a and/or b. Secondly, one can always test it:

>>> for a, b in [(10, 3), (10, -3), (-10, 3), (-10, -3)]:
...     assert ceil(a / b) == -(-a // b)

Here's the original tweet that taught me this:

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