Another way:
import math
def hextobinary(hex_string):
s = int(hex_string, 16)
num_digits = int(math.ceil(math.log(s) / math.log(2)))
digit_lst = ['0'] * num_digits
idx = num_digits
while s > 0:
idx -= 1
if s % 2 == 1: digit_lst[idx] = '1'
s = s / 2
return ''.join(digit_lst)
print hextobinary('abc123efff')