day10-1.py

s = '1321131112'

for n in xrange(0, 40):
    chains = []
    chain = s[0]
    prev = s[0]
    for index, c in enumerate(s[1:], start=1):
        if c == prev:
            chain += c
        else:
            chains.append(chain)
            chain = c
            prev = c
        if index == len(s[1:]):
            chains.append(chain)

    new_s = ''
    for item in chains:
        new_s += '{}{}'.format(len(item), item[0])
    s = new_s

print len(s)