import os
from pprint import pprint


source = '/home/wintermute/tmp'
mod_dates = [(file, os.path.getmtime(file)) for file in os.listdir(source)]
sorted_files = sorted(mod_dates, key=lambda x: x[1], reverse=True)
enum_files = dict([(file[0], index) for index, file in enumerate(sorted_files)])
for fname in os.listdir(source):
    os.rename(fname, "{0}_{1}".format(enum_files[fname], fname))