Code golf with Markov string generation

markov.py

import random


def build_lookup(corpus, key_size=2):
    lookup = {}
    for i, _ in enumerate(corpus):
        if i + key_size < len(corpus):
            words = lookup.setdefault(tuple(corpus[i:i + key_size]), [])
            words.append(corpus[i + key_size])
    return lookup


def generate_text(lookup, should_end=lambda words: False):
    words = list(random.choice([key for key in lookup if key[0][0].isupper()]))
    key_size = len(words)
    while True:
        key = tuple(words[-key_size:])
        if key not in lookup or should_end(words):
            break
        words.append(random.choice(lookup[key]))
    return ' '.join(words)


with open('corpus.txt', 'r') as f:
  lookup = build_lookup(f.read().split())
print(generate_text(lookup, should_end=lambda words: len(words) > 25))