# Sequence-to-sequence example in Keras (character-level).

This script demonstrates how to implement a basic character-level CNN sequence-to-sequence model. We apply it to translating short English sentences into short French sentences, character-by-character. Note that it is fairly unusual to do character-level machine translation, as word-level models are much more common in this domain. This example is for demonstration purposes only.

Summary of the algorithm

- We start with input sequences from a domain (e.g. English sentences) and corresponding target sequences from another domain (e.g. French sentences).
- An encoder CNN encodes the input character sequence.
- A decoder CNN is trained to turn the target sequences into the same sequence but offset by one timestep in the future, a training process called "teacher forcing" in this context. It uses the output from the encoder. Effectively, the decoder learns to generate `targets[t+1...]` given `targets[...t]`, conditioned on the input sequence.
- In inference mode, when we want to decode unknown input sequences, we:
  - Encode the input sequence.
  - Start with a target sequence of size 1 (just the start-of-sequence character)
  - Feed the input sequence and 1-char target sequence to the decoder to produce predictions for the next character
  - Sample the next character using these predictions (we simply use argmax).
  - Append the sampled character to the target sequence
  - Repeat until we hit the character limit.

Data download

English to French sentence pairs.

Lots of neat sentence pairs datasets.

References

- lstm_seq2seq.py
- https://wanasit.github.io/attention-based-sequence-to-sequence-in-keras.html

    from __future__ import print_function

    import numpy as np

    from keras.layers import Input, Convolution1D, Dot, Dense, Activation, Concatenate
    from keras.models import Model

    batch_size = 64  # Batch size for training.
    epochs = 100  # Number of epochs to train for.
    num_samples = 10000  # Number of samples to train on.
    # Path to the data txt file on disk.
    data_path = 'fra-eng/fra.txt'

    # Vectorize the data.
    input_texts = []
    target_texts = []
    input_characters = set()
    target_characters = set()
    with open(data_path, 'r', encoding='utf-8') as f:
        lines = f.read().split('\n')
    for line in lines[: min(num_samples, len(lines) - 1)]:
        input_text, target_text = line.split('\t')
        # We use "tab" as the "start sequence" character
        # for the targets, and "\n" as "end sequence" character.
        target_text = '\t' + target_text + '\n'
        input_texts.append(input_text)
        target_texts.append(target_text)
        for char in input_text:
            if char not in input_characters:
                input_characters.add(char)
        for char in target_text:
            if char not in target_characters:
                target_characters.add(char)

    input_characters = sorted(list(input_characters))
    target_characters = sorted(list(target_characters))
    num_encoder_tokens = len(input_characters)
    num_decoder_tokens = len(target_characters)
    max_encoder_seq_length = max([len(txt) for txt in input_texts])
    max_decoder_seq_length = max([len(txt) for txt in target_texts])

    print('Number of samples:', len(input_texts))
    print('Number of unique input tokens:', num_encoder_tokens)
    print('Number of unique output tokens:', num_decoder_tokens)
    print('Max sequence length for inputs:', max_encoder_seq_length)
    print('Max sequence length for outputs:', max_decoder_seq_length)

    input_token_index = dict(
        [(char, i) for i, char in enumerate(input_characters)])
    target_token_index = dict(
        [(char, i) for i, char in enumerate(target_characters)])

    encoder_input_data = np.zeros(
        (len(input_texts), max_encoder_seq_length, num_encoder_tokens),
        dtype='float32')
    decoder_input_data = np.zeros(
        (len(input_texts), max_decoder_seq_length, num_decoder_tokens),
        dtype='float32')
    decoder_target_data = np.zeros(
        (len(input_texts), max_decoder_seq_length, num_decoder_tokens),
        dtype='float32')

    for i, (input_text, target_text) in enumerate(zip(input_texts, target_texts)):
        for t, char in enumerate(input_text):
            encoder_input_data[i, t, input_token_index[char]] = 1.
        for t, char in enumerate(target_text):
            # decoder_target_data is ahead of decoder_input_data by one timestep
            decoder_input_data[i, t, target_token_index[char]] = 1.
            if t > 0:
                # decoder_target_data will be ahead by one timestep
                # and will not include the start character.
                decoder_target_data[i, t - 1, target_token_index[char]] = 1.

    # Define an input sequence and process it.
    encoder_inputs = Input(shape=(None, num_encoder_tokens))
    # Encoder
    x_encoder = Convolution1D(256, kernel_size=3, activation='relu',
                              padding='causal')(encoder_inputs)
    x_encoder = Convolution1D(256, kernel_size=3, activation='relu',
                              padding='causal', dilation_rate=2)(x_encoder)
    x_encoder = Convolution1D(256, kernel_size=3, activation='relu',
                              padding='causal', dilation_rate=4)(x_encoder)

    decoder_inputs = Input(shape=(None, num_decoder_tokens))
    # Decoder
    x_decoder = Convolution1D(256, kernel_size=3, activation='relu',
                              padding='causal')(decoder_inputs)
    x_decoder = Convolution1D(256, kernel_size=3, activation='relu',
                              padding='causal', dilation_rate=2)(x_decoder)
    x_decoder = Convolution1D(256, kernel_size=3, activation='relu',
                              padding='causal', dilation_rate=4)(x_decoder)
    # Attention
    attention = Dot(axes=[2, 2])([x_decoder, x_encoder])
    attention = Activation('softmax')(attention)

    context = Dot(axes=[2, 1])([attention, x_encoder])
    decoder_combined_context = Concatenate(axis=-1)([context, x_decoder])

    decoder_outputs = Convolution1D(64, kernel_size=3, activation='relu',
                                    padding='causal')(decoder_combined_context)
    decoder_outputs = Convolution1D(64, kernel_size=3, activation='relu',
                                    padding='causal')(decoder_outputs)
    # Output
    decoder_dense = Dense(num_decoder_tokens, activation='softmax')
    decoder_outputs = decoder_dense(decoder_outputs)

    # Define the model that will turn
    # `encoder_input_data` & `decoder_input_data` into `decoder_target_data`
    model = Model([encoder_inputs, decoder_inputs], decoder_outputs)
    model.summary()

    # Run training
    model.compile(optimizer='adam', loss='categorical_crossentropy')
    model.fit([encoder_input_data, decoder_input_data], decoder_target_data,
              batch_size=batch_size,
              epochs=epochs,
              validation_split=0.2)
    # Save model
    model.save('cnn_s2s.h5')

    # Next: inference mode (sampling).

    # Define sampling models
    reverse_input_char_index = dict(
        (i, char) for char, i in input_token_index.items())
    reverse_target_char_index = dict(
        (i, char) for char, i in target_token_index.items())

    nb_examples = 100
    in_encoder = encoder_input_data[:nb_examples]
    in_decoder = np.zeros(
        (len(input_texts), max_decoder_seq_length, num_decoder_tokens),
        dtype='float32')

    in_decoder[:, 0, target_token_index["\t"]] = 1

    predict = np.zeros(
        (len(input_texts), max_decoder_seq_length),
        dtype='float32')

    for i in range(max_decoder_seq_length - 1):
        predict = model.predict([in_encoder, in_decoder])
        predict = predict.argmax(axis=-1)
        predict_ = predict[:, i].ravel().tolist()
        for j, x in enumerate(predict_):
            in_decoder[j, i + 1, x] = 1

    for seq_index in range(nb_examples):
        # Take one sequence (part of the training set)
        # for trying out decoding.
        output_seq = predict[seq_index, :].ravel().tolist()
        decoded = []
        for x in output_seq:
            if reverse_target_char_index[x] == "\n":
                break
            else:
                decoded.append(reverse_target_char_index[x])
        decoded_sentence = "".join(decoded)
        print('-')
        print('Input sentence:', input_texts[seq_index])
        print('Decoded sentence:', decoded_sentence)

GitHub