A minor detail to note is the difference between the "+=" and "append" when it comes to Python lists. In many applications the two are interchangeable, but here they are not. If you are appending a list of lists to another list of lists, "append" will only append the first list; you need to use "+=" in order to join all of the lists at once.

下面的例子中：review_to_sentences(review,tokenizer)返回值是一個list of lists

sentences = []  # Initialize an empty list of sentences

print "Parsing sentences from training set"
for review in train["review"]:
    sentences += review_to_sentences(review, tokenizer)

print "Parsing sentences from unlabeled set"
for review in unlabeled_train["review"]:
    sentences += review_to_sentences(review, tokenizer)