參考官方tutorial
https://docs.python.org/3.5/tutorial/datastructures.html

在moviereview分類中，看到
word_index = {k:(v+3) for k,v in word_index.items()}
對dctionary不熟，看官方turorial，寫小測試

b={‘a’:0,‘b’:1,‘c’:2}
b[‘d’]=3
print(b)
c={k:(v+3) for k,v in b.items()}
print©

{‘d’: 3, ‘c’: 2, ‘b’: 1, ‘a’: 0}
{‘d’: 6, ‘c’: 5, ‘b’: 4, ‘a’: 3}

看回程式
原本的字典資料如圖

經過

#A dictionary mapping words to an integer index
word_index = imdb.get_word_index()
#The first indices are reserved
word_index = {k:(v+3) for k,v in word_index.items()}
word_index[""] = 0
word_index[""] = 1
word_index[""] = 2 # unknown
word_index[""] = 3

字典資料變為

便於理解該字典內容
後面很好理解

reverse_word_index = dict([(value, key) for (key, value) in word_index.items()])

後面

def decode_review(text):
return ’ '.join([reverse_word_index.get(i, ‘?’) for i in text])

解碼，將“數字”按照字典轉換為“文字”