Keras 3 API documentation / Built-in small datasets / IMDB movie review sentiment classification dataset

IMDB movie review sentiment classification dataset


load_data function

Loads the IMDB dataset.

This is a dataset of 25,000 movies reviews from IMDB, labeled by sentiment (positive/negative). Reviews have been preprocessed, and each review is encoded as a list of word indexes (integers). For convenience, words are indexed by overall frequency in the dataset, so that for instance the integer "3" encodes the 3rd most frequent word in the data. This allows for quick filtering operations such as: "only consider the top 10,000 most common words, but eliminate the top 20 most common words".

As a convention, "0" does not stand for a specific word, but instead is used to encode the pad token.


  • path: where to cache the data (relative to ~/.keras/dataset).
  • num_words: integer or None. Words are ranked by how often they occur (in the training set) and only the num_words most frequent words are kept. Any less frequent word will appear as oov_char value in the sequence data. If None, all words are kept. Defaults to None.
  • skip_top: skip the top N most frequently occurring words (which may not be informative). These words will appear as oov_char value in the dataset. When 0, no words are skipped. Defaults to 0.
  • maxlen: int or None. Maximum sequence length. Any longer sequence will be truncated. None, means no truncation. Defaults to None.
  • seed: int. Seed for reproducible data shuffling.
  • start_char: int. The start of a sequence will be marked with this character. 0 is usually the padding character. Defaults to 1.
  • oov_char: int. The out-of-vocabulary character. Words that were cut out because of the num_words or skip_top limits will be replaced with this character.
  • index_from: int. Index actual words with this index and higher.


  • Tuple of Numpy arrays: (x_train, y_train), (x_test, y_test).

x_train, x_test: lists of sequences, which are lists of indexes (integers). If the num_words argument was specific, the maximum possible index value is num_words - 1. If the maxlen argument was specified, the largest possible sequence length is maxlen.

y_train, y_test: lists of integer labels (1 or 0).

Note: The 'out of vocabulary' character is only used for words that were present in the training set but are not included because they're not making the num_words cut here. Words that were not seen in the training set but are in the test set have simply been skipped.


get_word_index function"imdb_word_index.json")

Retrieves a dict mapping words to their index in the IMDB dataset.


  • path: where to cache the data (relative to ~/.keras/dataset).


The word index dictionary. Keys are word strings, values are their index.


# Use the default parameters to
start_char = 1
oov_char = 2
index_from = 3
# Retrieve the training sequences.
(x_train, _), _ =
    start_char=start_char, oov_char=oov_char, index_from=index_from
# Retrieve the word index file mapping words to indices
word_index =
# Reverse the word index to obtain a dict mapping indices to words
# And add `index_from` to indices to sync with `x_train`
inverted_word_index = dict(
    (i + index_from, word) for (word, i) in word_index.items()
# Update `inverted_word_index` to include `start_char` and `oov_char`
inverted_word_index[start_char] = "[START]"
inverted_word_index[oov_char] = "[OOV]"
# Decode the first sequence in the dataset
decoded_sequence = " ".join(inverted_word_index[i] for i in x_train[0])