day3.py

#!/bin/python

with open('day3.txt') as f:
    bitslist = [list(map(int, line.strip())) for line in f]

def column(bitslist, location):
    return [bits[location] for bits in bitslist]

def bitcount(bitslist, location):
    zero_count = column(bitslist, location).count(0)
    one_count = len(bitslist) - zero_count
    return [zero_count, one_count]

def all_bitscount(bitslist):
    return [bitcount(bitslist, i) for i in range(len(bitslist[0]))]

def to_int(binary):
    return int(''.join(map(str, binary)), 2)

overall = all_bitscount(bitslist)
gamma   = [1 if v0 > v1 else 0 for v0, v1 in overall]
epsilon = [1 if v0 < v1 else 0 for v0, v1 in overall]

print('Part One:', to_int(gamma) * to_int(epsilon))

def value_at_position(i, bit):
    return lambda bits: bits[i] == bit

def getvalues(value, condition):
    for i in range(len(value[0])):
        count = bitcount(value, i)
        wanted_bit = condition(count[1],count[0])
        value = list(filter(value_at_position(i, wanted_bit), value))
        if len(value) == 1:
            return value[0]

oxy = getvalues(bitslist, lambda a, b: 1 if a >= b else 0)
co2 = getvalues(bitslist, lambda a, b: 0 if b <= a else 1)

print('Part Two:',to_int(oxy)*to_int(co2))

credits to @Tomalak who helped me a lot with that, and answered all my stupid questions :)