Given a list of 10k 32-bit integers, calculate the number of set bits in each. Quickly.

bits.py

import random

data = list()

for i in range(10000):
    data.append(random.randint(0, 2**32))

bits_set = {0: 0}
for i in range(256):
    bits_set[i] = (i & 1) + bits_set[i / 2]


def count_ones(x):
    return (
        bits_set[x & 255] + bits_set[(x >> 8) & 255] +
        bits_set[(x >> 16) & 255] + bits_set[(x >> 24) & 255])

for x in data:
    print '%s: %s: %s' % (x, bin(x), count_ones(x))