How to Develop a Character-Based Neural Language Model in Keras

 A language model predicts the next word in the sequence based on the specific words that have come before it in the sequence.

It is also possible to develop language models at the character level using neural networks. The benefit of character-based language models is their small vocabulary and flexibility in handling any words, punctuation, and other document structure. This comes at the cost of requiring larger models that are slower to train.

Nevertheless, in the field of neural language models, character-based models offer a lot of promise for a general, flexible and powerful approach to language modeling.

In this tutorial, you will discover how to develop a character-based neural language model.

After completing this tutorial, you will know:

• How to prepare text for character-based language modeling.
• How to develop a character-based language model using LSTMs.
• How to use a trained character-based language model to generate text.

Kick-start your project with my new book Deep Learning for Natural Language Processing, including step-by-step tutorials and the Python source code files for all examples.

Let’s get started.

• Update Feb/2018: Minor update to generation for API change in Keras 2.1.3.
• Update Feb/2021: Updated final code example to remove redundant line.

How to Develop a Character-Based Neural Language Model in Keras
Photo by hedera.baltica, some rights reserved.

Tutorial Overview

This tutorial is divided into 4 parts; they are:

1. Sing a Song of Sixpence
2. Data Preparation
3. Train Language Model
4. Generate Text

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Sing a Song of Sixpence

The nursery rhyme “Sing a Song of Sixpence” is well known in the west.

The first verse is common, but there is also a 4 verse version that we will use to develop our character-based language model.

It is short, so fitting the model will be fast, but not so short that we won’t see anything interesting.

The complete 4 verse version we will use as source text is listed below.

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Sing a song of sixpence, A pocket full of rye. Four and twenty blackbirds, Baked in a pie.   When the pie was opened The birds began to sing; Wasn't that a dainty dish, To set before the king.   The king was in his counting house, Counting out his money; The queen was in the parlour, Eating bread and honey.   The maid was in the garden, Hanging out the clothes, When down came a blackbird And pecked off her nose.

Copy the text and save it in a new file in your current working directory with the file name ‘rhyme.txt‘.

Data Preparation

The first step is to prepare the text data.

We will start by defining the type of language model.

Language Model Design

A language model must be trained on the text, and in the case of a character-based language model, the input and output sequences must be characters.

The number of characters used as input will also define the number of characters that will need to be provided to the model in order to elicit the first predicted character.

After the first character has been generated, it can be appended to the input sequence and used as input for the model to generate the next character.

Longer sequences offer more context for the model to learn what character to output next but take longer to train and impose more burden on seeding the model when generating text.

We will use an arbitrary length of 10 characters for this model.

There is not a lot of text, and 10 characters is a few words.

We can now transform the raw text into a form that our model can learn; specifically, input and output sequences of characters.

Load Text

We must load the text into memory so that we can work with it.

Below is a function named load_doc() that will load a text file given a filename and return the loaded text.

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# load doc into memory def load_doc(filename): # open the file as read only file = open(filename, 'r') # read all text text = file.read() # close the file file.close() return text

We can call this function with the filename of the nursery rhyme ‘rhyme.txt‘ to load the text into memory. The contents of the file are then printed to screen as a sanity check.

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# load text raw_text = load_doc('rhyme.txt') print(raw_text)

Clean Text

Next, we need to clean the loaded text.

We will not do much to it here. Specifically, we will strip all of the new line characters so that we have one long sequence of characters separated only by white space.

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# clean tokens = raw_text.split() raw_text = ' '.join(tokens)

You may want to explore other methods for data cleaning, such as normalizing the case to lowercase or removing punctuation in an effort to reduce the final vocabulary size and develop a smaller and leaner model.

Create Sequences

Now that we have a long list of characters, we can create our input-output sequences used to train the model.

Each input sequence will be 10 characters with one output character, making each sequence 11 characters long.

We can create the sequences by enumerating the characters in the text, starting at the 11th character at index 10.

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# organize into sequences of characters length = 10 sequences = list() for i in range(length, len(raw_text)): # select sequence of tokens seq = raw_text[i-length:i+1] # store sequences.append(seq) print('Total Sequences: %d' % len(sequences))

Running this snippet, we can see that we end up with just under 400 sequences of characters for training our language model.

1	Total Sequences: 399

Save Sequences

Finally, we can save the prepared data to file so that we can load it later when we develop our model.

Below is a function save_doc() that, given a list of strings and a filename, will save the strings to file, one per line.

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# save tokens to file, one dialog per line def save_doc(lines, filename): data = '\n'.join(lines) file = open(filename, 'w') file.write(data) file.close()

We can call this function and save our prepared sequences to the filename ‘char_sequences.txt‘ in our current working directory.

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# save sequences to file out_filename = 'char_sequences.txt' save_doc(sequences, out_filename)

Complete Example

Tying all of this together, the complete code listing is provided below.

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# load doc into memory def load_doc(filename): # open the file as read only file = open(filename, 'r') # read all text text = file.read() # close the file file.close() return text   # save tokens to file, one dialog per line def save_doc(lines, filename): data = '\n'.join(lines) file = open(filename, 'w') file.write(data) file.close()   # load text raw_text = load_doc('rhyme.txt') print(raw_text)   # clean tokens = raw_text.split() raw_text = ' '.join(tokens)   # organize into sequences of characters length = 10 sequences = list() for i in range(length, len(raw_text)): # select sequence of tokens seq = raw_text[i-length:i+1] # store sequences.append(seq) print('Total Sequences: %d' % len(sequences))   # save sequences to file out_filename = 'char_sequences.txt' save_doc(sequences, out_filename)

Run the example to create the ‘char_seqiences.txt‘ file.

Take a look inside you should see something like the following:

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Sing a song ing a song ng a song o g a song of a song of a song of s song of si song of six ong of sixp ng of sixpe ...

We are now ready to train our character-based neural language model.

Train Language Model

In this section, we will develop a neural language model for the prepared sequence data.

The model will read encoded characters and predict the next character in the sequence. A Long Short-Term Memory recurrent neural network hidden layer will be used to learn the context from the input sequence in order to make the predictions.

Load Data

The first step is to load the prepared character sequence data from ‘char_sequences.txt‘.

We can use the same load_doc() function developed in the previous section. Once loaded, we split the text by new line to give a list of sequences ready to be encoded.

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# load doc into memory def load_doc(filename): # open the file as read only file = open(filename, 'r') # read all text text = file.read() # close the file file.close() return text   # load in_filename = 'char_sequences.txt' raw_text = load_doc(in_filename) lines = raw_text.split('\n')

Encode Sequences

The sequences of characters must be encoded as integers.

This means that each unique character will be assigned a specific integer value and each sequence of characters will be encoded as a sequence of integers.

We can create the mapping given a sorted set of unique characters in the raw input data. The mapping is a dictionary of character values to integer values.

1 2	chars = sorted(list(set(raw_text))) mapping = dict((c, i) for i, c in enumerate(chars))

Next, we can process each sequence of characters one at a time and use the dictionary mapping to look up the integer value for each character.

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sequences = list() for line in lines: # integer encode line encoded_seq = [mapping[char] for char in line] # store sequences.append(encoded_seq)

The result is a list of integer lists.

We need to know the size of the vocabulary later. We can retrieve this as the size of the dictionary mapping.

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# vocabulary size vocab_size = len(mapping) print('Vocabulary Size: %d' % vocab_size)

Running this piece, we can see that there are 38 unique characters in the input sequence data.

1	Vocabulary Size: 38

Split Inputs and Output

Now that the sequences have been integer encoded, we can separate the columns into input and output sequences of characters.

We can do this using a simple array slice.

1 2	sequences = array(sequences) X, y = sequences[:,:-1], sequences[:,-1]

Next, we need to one hot encode each character. That is, each character becomes a vector as long as the vocabulary (38 elements) with a 1 marked for the specific character. This provides a more precise input representation for the network. It also provides a clear objective for the network to predict, where a probability distribution over characters can be output by the model and compared to the ideal case of all 0 values with a 1 for the actual next character.

We can use the to_categorical() function in the Keras API to one hot encode the input and output sequences.

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sequences = [to_categorical(x, num_classes=vocab_size) for x in X] X = array(sequences) y = to_categorical(y, num_classes=vocab_size)

We are now ready to fit the model.

Fit Model

The model is defined with an input layer that takes sequences that have 10 time steps and 38 features for the one hot encoded input sequences.

Rather than specify these numbers, we use the second and third dimensions on the X input data. This is so that if we change the length of the sequences or size of the vocabulary, we do not need to change the model definition.

The model has a single LSTM hidden layer with 75 memory cells, chosen with a little trial and error.

The model has a fully connected output layer that outputs one vector with a probability distribution across all characters in the vocabulary. A softmax activation function is used on the output layer to ensure the output has the properties of a probability distribution.

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# define model model = Sequential() model.add(LSTM(75, input_shape=(X.shape[1], X.shape[2]))) model.add(Dense(vocab_size, activation='softmax')) print(model.summary())

Running this prints a summary of the defined network as a sanity check.

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_________________________________________________________________ Layer (type)                 Output Shape              Param # ================================================================= lstm_1 (LSTM)                (None, 75)                34200 _________________________________________________________________ dense_1 (Dense)              (None, 38)                2888 ================================================================= Total params: 37,088 Trainable params: 37,088 Non-trainable params: 0 _________________________________________________________________

The model is learning a multi-class classification problem, therefore we use the categorical log loss intended for this type of problem. The efficient Adam implementation of gradient descent is used to optimize the model and accuracy is reported at the end of each batch update.

The model is fit for 100 training epochs, again found with a little trial and error.

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# compile model model.compile(loss='categorical_crossentropy', optimizer='adam', metrics=['accuracy']) # fit model model.fit(X, y, epochs=100, verbose=2)

Save Model

After the model is fit, we save it to file for later use.

The Keras model API provides the save() function that we can use to save the model to a single file, including weights and topology information.

1 2	# save the model to file model.save('model.h5')

We also save the mapping from characters to integers that we will need to encode any input when using the model and decode any output from the model.

1 2	# save the mapping dump(mapping, open('mapping.pkl', 'wb'))

Complete Example

Tying all of this together, the complete code listing for fitting the character-based neural language model is listed below.

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from numpy import array from pickle import dump from keras.utils import to_categorical from keras.models import Sequential from keras.layers import Dense from keras.layers import LSTM   # load doc into memory def load_doc(filename): # open the file as read only file = open(filename, 'r') # read all text text = file.read() # close the file file.close() return text   # load in_filename = 'char_sequences.txt' raw_text = load_doc(in_filename) lines = raw_text.split('\n')   # integer encode sequences of characters chars = sorted(list(set(raw_text))) mapping = dict((c, i) for i, c in enumerate(chars)) sequences = list() for line in lines: # integer encode line encoded_seq = [mapping[char] for char in line] # store sequences.append(encoded_seq)   # vocabulary size vocab_size = len(mapping) print('Vocabulary Size: %d' % vocab_size)   # separate into input and output sequences = array(sequences) X, y = sequences[:,:-1], sequences[:,-1] sequences = [to_categorical(x, num_classes=vocab_size) for x in X] X = array(sequences) y = to_categorical(y, num_classes=vocab_size)   # define model model = Sequential() model.add(LSTM(75, input_shape=(X.shape[1], X.shape[2]))) model.add(Dense(vocab_size, activation='softmax')) print(model.summary()) # compile model model.compile(loss='categorical_crossentropy', optimizer='adam', metrics=['accuracy']) # fit model model.fit(X, y, epochs=100, verbose=2)   # save the model to file model.save('model.h5') # save the mapping dump(mapping, open('mapping.pkl', 'wb'))

Running the example might take one minute.

Note: Your results may vary given the stochastic nature of the algorithm or evaluation procedure, or differences in numerical precision. Consider running the example a few times and compare the average outcome.

You will see that the model learns the problem well, perhaps too well for generating surprising sequences of characters.

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... Epoch 96/100 0s - loss: 0.2193 - acc: 0.9950 Epoch 97/100 0s - loss: 0.2124 - acc: 0.9950 Epoch 98/100 0s - loss: 0.2054 - acc: 0.9950 Epoch 99/100 0s - loss: 0.1982 - acc: 0.9950 Epoch 100/100 0s - loss: 0.1910 - acc: 0.9950

At the end of the run, you will have two files saved to the current working directory, specifically model.h5 and mapping.pkl.

Next, we can look at using the learned model.

Generate Text

We will use the learned language model to generate new sequences of text that have the same statistical properties.

Load Model

The first step is to load the model saved to the file ‘model.h5‘.

We can use the load_model() function from the Keras API.

1 2	# load the model model = load_model('model.h5')

We also need to load the pickled dictionary for mapping characters to integers from the file ‘mapping.pkl‘. We will use the Pickle API to load the object.

1 2	# load the mapping mapping = load(open('mapping.pkl', 'rb'))

We are now ready to use the loaded model.

Generate Characters

We must provide sequences of 10 characters as input to the model in order to start the generation process. We will pick these manually.

A given input sequence will need to be prepared in the same way as preparing the training data for the model.

First, the sequence of characters must be integer encoded using the loaded mapping.

1 2	# encode the characters as integers encoded = [mapping[char] for char in in_text]

Next, the sequences need to be one hot encoded using the to_categorical() Keras function.

1 2	# one hot encode encoded = to_categorical(encoded, num_classes=len(mapping))

We can then use the model to predict the next character in the sequence.

We use predict_classes() instead of predict() to directly select the integer for the character with the highest probability instead of getting the full probability distribution across the entire set of characters.

1 2	# predict character yhat = model.predict_classes(encoded, verbose=0)

We can then decode this integer by looking up the mapping to see the character to which it maps.

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out_char = '' for char, index in mapping.items(): if index == yhat: out_char = char break

This character can then be added to the input sequence. We then need to make sure that the input sequence is 10 characters by truncating the first character from the input sequence text.

We can use the pad_sequences() function from the Keras API that can perform this truncation operation.

Putting all of this together, we can define a new function named generate_seq() for using the loaded model to generate new sequences of text.

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# generate a sequence of characters with a language model def generate_seq(model, mapping, seq_length, seed_text, n_chars): in_text = seed_text # generate a fixed number of characters for _ in range(n_chars): # encode the characters as integers encoded = [mapping[char] for char in in_text] # truncate sequences to a fixed length encoded = pad_sequences([encoded], maxlen=seq_length, truncating='pre') # one hot encode encoded = to_categorical(encoded, num_classes=len(mapping)) # predict character yhat = model.predict_classes(encoded, verbose=0) # reverse map integer to character out_char = '' for char, index in mapping.items(): if index == yhat: out_char = char break # append to input in_text += char return in_text

Complete Example

Tying all of this together, the complete example for generating text using the fit neural language model is listed below.

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from pickle import load from keras.models import load_model from keras.utils import to_categorical from keras.preprocessing.sequence import pad_sequences   # generate a sequence of characters with a language model def generate_seq(model, mapping, seq_length, seed_text, n_chars): in_text = seed_text # generate a fixed number of characters for _ in range(n_chars): # encode the characters as integers encoded = [mapping[char] for char in in_text] # truncate sequences to a fixed length encoded = pad_sequences([encoded], maxlen=seq_length, truncating='pre') # one hot encode encoded = to_categorical(encoded, num_classes=len(mapping)) # predict character yhat = model.predict_classes(encoded, verbose=0) # reverse map integer to character out_char = '' for char, index in mapping.items(): if index == yhat: out_char = char break # append to input in_text += char return in_text   # load the model model = load_model('model.h5') # load the mapping mapping = load(open('mapping.pkl', 'rb'))   # test start of rhyme print(generate_seq(model, mapping, 10, 'Sing a son', 20)) # test mid-line print(generate_seq(model, mapping, 10, 'king was i', 20)) # test not in original print(generate_seq(model, mapping, 10, 'hello worl', 20))

Running the example generates three sequences of text.

Note: Your results may vary given the stochastic nature of the algorithm or evaluation procedure, or differences in numerical precision. Consider running the example a few times and compare the average outcome.

The first is a test to see how the model does at starting from the beginning of the rhyme. The second is a test to see how well it does at beginning in the middle of a line. The final example is a test to see how well it does with a sequence of characters never seen before.

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Sing a song of sixpence, A poc king was in his counting house hello worls e pake wofey. The

We can see that the model did very well with the first two examples, as we would expect. We can also see that the model still generated something for the new text, but it is nonsense.

Extensions

This section lists some ideas for extending the tutorial that you may wish to explore.

• Padding. Update the example to provides sequences line by line only and use padding to fill out each sequence to the maximum line length.
• Sequence Length. Experiment with different sequence lengths and see how they impact the behavior of the model.
• Tune Model. Experiment with different model configurations, such as the number of memory cells and epochs, and try to develop a better model for fewer resources.

Further Reading

This section provides more resources on the topic if you are looking go deeper.

• Sing a Song of Sixpence on Wikipedia
• Text Generation With LSTM Recurrent Neural Networks in Python with Keras
• Keras Utils API
• Keras Sequence Processing API

Summary

In this tutorial, you discovered how to develop a character-based neural language model.

Specifically, you learned:

• How to prepare text for character-based language modeling.
• How to develop a character-based language model using LSTMs.
• How to use a trained character-based language model to generate text.

Do you have any questions?
Ask your questions in the comments below and I will do my best to answer.