log2 in python math module

Question

why doesn't it exist?

import math [x for x in dir(math) if 'log' in x] >>> ['log', 'log10', 'log1p'] 

I know I can do log(x,2), but log2 is really common, so I'm kind of baffled.

Oh, it looks like it's only defined in C99, not C90, I guess that answers my question. Still seems kind of silly.

Answer 1

I think you've answered your own question. :-) There's no log2(x) because you can do log(x, 2). As The Zen of Python (PEP 20) says, "There should be one-- and preferably only one --obvious way to do it."

That said, log2 was considered in Issue3366 (scroll down to the last 3 messages) which added several other C99 math functions to the math module for Python 2.7 and 3.2.

Edit: log2 was reconsidered in Issue11888 and added in Python 3.3.

Answer 2

I'm not sure that there is that you want, but:

-- From math point of view you can do for example math.log(x)/math.log(2).

-- If input X has an integral type and you are waiting for the integral rounded result - you can do it rather faster with right shifting. This works with SHR command and without Taylor series + local interpolation, which is under the hood of libc log() calls.