Text generator

"""Case-study #7 Генерация предложений
Разработчики:
Дарбаа Ч.Ю.

"""
import random
import re
def text_generator(filename, n_senstences = 20):
    input_file = open(filename, 'r')
    text = input_file.read()
    text = text.replace("\n", " ")
    text = re.sub('[^a-zA-Zа-яА-Я" "0-9 !.,?]+', '', text)
    text = re.sub(' +', ' ', text)
    text = text.replace(' ?', '?').replace(' !', '!').replace(' .', '.').replace(' ,', ',')
    words = text.split()
    chain = {}
    for i in range(len(words) - 1):
        if words[i] in chain.keys():
            chain[words[i]].append(words[i + 1])
        else:
            chain[words[i]] = [words[i + 1]]
    capital = 'ЁЙЦУКЕНГШЩЗХЪФЫВАПРОЛДЖЭЯЧСМИТЬБЮQWERTYUIOPASDFGHJKLZXCVBNM'
    start_words = [word for word in words if word[0] in capital]
    start_words = list(set(start_words))  #optional
    generated_text = ''
    sentence_end = '.!?'
    for i in range(n_senstences):
        chain_word = random.choice(start_words)
        generated_text += chain_word + ' '
        num_words = random.randint(4, 19)
        for j in range(num_words):
            if chain_word in chain.keys():
                chain_word = random.choice(chain[chain_word])
            else:
                chain_word = random.choice(words)
            generated_text += chain_word + ' '
        if generated_text[-2] == ',':
            generated_text = generated_text[:-2] + random.choice(sentence_end) + " "
        elif generated_text[-2] not in sentence_end:
            generated_text = generated_text[:-1] + random.choice(sentence_end) + " "
    return generated_text

print(text_generator('text.txt', 5))