Markov Chain text generator in Python

import random
def create_chain_arr_lines(lines):
    markov_chain = {}
    hasPrev = False
    for line in lines:
        for currWord in line.split():
            if currWord != '':
                currWord = currWord.lower()
                if hasPrev == False:
                    prevWord = currWord
                    hasPrev = True
                else:
                    markov_chain.setdefault(prevWord, []).append(currWord)
                    prevWord = currWord
    return markov_chain

def construct_sentence(markov_chain):
    while True:
        word = random.choice(markov_chain.keys())
        if word[-1] not in ('.','?'):
            break
    generated_sentence = word.capitalize()
    while word[-1] not in ('.','?'):
        newword = random.choice(markov_chain[word])
        generated_sentence += ' '+newword
        word = newword #TODO fix possible crash if this is not a key (last word parsed)
    return generated_sentence

#example usage
mc = create_chain_arr_lines(open('someLargeText.txt'))

#Generate 20 phrases
for i in xrange(0, 20):
    print construct_sentence(mc)