jesse.py

#!/usr/bin/env python

import collections

def main():
    di = {}
    f = file('in.txt', 'r')
    for line in f:
        k = line.rstrip().split(',')[0]
        v = line.rstrip().split(',')[1]

        try:
            di[k].append(v)
        except KeyError:
            di[k] = [v]

    f = file('out.txt', 'w')
    od = collections.OrderedDict(sorted(di.items()))
    for k, v in od.items():
        #print k, v
        s = k + ',' 
        for el in v:
            s += el + ';'
        if s[-1] == ';':
            s = s[:-1]
        s += '\n'
        f.write(s)

if __name__ == '__main__':
    main()