Word generation using markov chains

words.py

from sys import argv
from collections import defaultdict
from random import choice


class DefaultList(list):
    def __getitem__(self, idx):
        try:
            return super().__getitem__(idx)
        except IndexError:
            return [None, 3, 1, ""][idx]

argv, dictionary = DefaultList(argv[:]), defaultdict(list)
prefix, count, start = int(argv[1]), int(argv[2]), str(argv[3])
for word in ("^%s$" % line.strip().lower() for line in open("/usr/share/dict/words")):
    while len(word) >= prefix + 1:
        dictionary[word[:prefix]].append(word[prefix])
        word = word[1:]

starters = list(k for k in dictionary.keys() if k[0:len(start) + 1] == "^%s" % start)
for word in [""] * count:
    while word == "" or word[-1] != "$":
        word += choice(starters if word == "" else dictionary[word[-prefix:]])
    print(word[:-1][1:])