random sentence generator in python not working

import random

articles = ("A", "THE")
nouns = ("BOY", "GIRL", "BAT", "BALL")
verbs = ("HIT", "SAW", "LIKED")
prepositions = ("WITH", "BY")

def sentence():
    return nounPhrase() + " " + verbPhrase()

def nounPhrase():
    return random.choice(articles) + " " + random.choice(nouns)

def verbPhrase():
    return random.choice(verbs) + "  " + nounPhrase() + " " + \
        prepositionalPhrase()

def prepositionalPhrase():
    return random.choice(prepositions) + " " + nounPhrase()

def main():
    number = int(input("Enter the number of sentences: "))
    for count in range(number):
         print(sentence())
main()

I wanna modify this sentence-generator program so that it inputs its vocabulary from a set of text files at startup. The filenames are nouns.txt, verbs.txt, articles.txt, and prepositions.txt. ***