The confidence sort in pure Python (from Reddit's codebase)

sorts.py

# Rewritten code from /r2/r2/lib/db/_sorts.pyx
from math import sqrt

def _confidence(ups, downs):
    n = ups + downs

    if n == 0:
        return 0

    z = 1.281551565545
    p = float(ups) / n

    left = p + 1/(2*n)*z*z
    right = z*sqrt(p*(1-p)/n + z*z/(4*n*n))
    under = 1+1/n*z*z

    return (left - right) / under

def confidence(ups, downs):
    if ups + downs == 0:
        return 0
    else:
        return _confidence(ups, downs)

def confidence(ups, downs):
    if ups + downs == 0:
        return 0

this is double work; because

def _confidence(ups, downs):
    n = ups + downs
    if n == 0:
        return 0

already does that.

you should remove confidence and rename _confidence to confidence
see my fork for a better understanding: https://gist.github.com/jay-bricksoft/fdd3797b1443153389d34a4b5280ac54
cheers

In python2 x/y is integer division, so line #13 and #15 will be incorrect.
Easily fixed by moving the z*z to the left of the division.