The core building blocks are RNN Encoder-Decoder architectures and Attention mechanism.

The package was largely implemented using the latest (1.2) tf.contrib.seq2seq modules

• AttentionWrapper
• Decoder
• BasicDecoder
• BeamSearchDecoder

The package supports

• Multi-layer GRU/LSTM
• Residual connection
• Dropout
• Attention and input_feeding
• Beamsearch decoding
• Write n-best list

• NumPy >= 1.11.1
• Tensorflow >= 1.2

• June 5, 2017: Major update
• June 6, 2017: Supports batch beamsearch decoding
• June 11, 2017: Separted training / decoding
• June 22, 2017: Supports tf.1.2 (contrib.rnn -> python.ops.rnn_cell)

To preprocess raw parallel data of sample_data.src and sample_data.trg, simply run

cd data/
./preprocess.sh src trg sample_data ${max_seq_len}

Running the above code performs widely used preprocessing steps for Machine Translation (MT).

• Normalizing punctuation
• Tokenizing
• Bytepair encoding (# merge = 30000) (Sennrich et al., 2016)
• Cleaning sequences of length over ${max_seq_len}
• Shuffling
• Building dictionaries

To train a seq2seq model,

$ python train.py   --cell_type 'lstm' \ 
                    --attention_type 'luong' \
                    --hidden_units 1024 \
                    --depth 2 \
                    --embedding_size 500 \
                    --num_encoder_symbols 30000 \
                    --num_decoder_symbols 30000 ...

To run the trained model for decoding,

$ python decode.py  --beam_width 5 \
                    --decode_batch_size 30 \
                    --model_path $PATH_TO_A_MODEL_CHECKPOINT (e.g. model/translate.ckpt-100) \
                    --max_decode_step 300 \
                    --write_n_best False
                    --decode_input $PATH_TO_DECODE_INPUT
                    --decode_output $PATH_TO_DECODE_OUTPUT
                    

If --beam_width=1, greedy decoding is performed at each time-step.

Data params

• --source_vocabulary : Path to source vocabulary
• --target_vocabulary : Path to target vocabulary
• --source_train_data : Path to source training data
• --target_train_data : Path to target training data
• --source_valid_data : Path to source validation data
• --target_valid_data : Path to target validation data

Network params

• --cell_type : RNN cell to use for encoder and decoder (default: lstm)
• --attention_type : Attention mechanism (bahdanau, luong), (default: bahdanau)
• --depth : Number of hidden units for each layer in the model (default: 2)
• --embedding_size : Embedding dimensions of encoder and decoder inputs (default: 500)
• --num_encoder_symbols : Source vocabulary size to use (default: 30000)
• --num_decoder_symbols : Target vocabulary size to use (default: 30000)
• --use_residual : Use residual connection between layers (default: True)
• --attn_input_feeding : Use input feeding method in attentional decoder (Luong et al., 2015) (default: True)
• --use_dropout : Use dropout in rnn cell output (default: True)
• --dropout_rate : Dropout probability for cell outputs (0.0: no dropout) (default: 0.3)

Training params

• --learning_rate : Number of hidden units for each layer in the model (default: 0.0002)
• --max_gradient_norm : Clip gradients to this norm (default 1.0)
• --batch_size : Batch size
• --max_epochs : Maximum training epochs
• --max_load_batches : Maximum number of batches to prefetch at one time.
• --max_seq_length : Maximum sequence length
• --display_freq : Display training status every this iteration
• --save_freq : Save model checkpoint every this iteration
• --valid_freq : Evaluate the model every this iteration: valid_data needed
• --optimizer : Optimizer for training: (adadelta, adam, rmsprop) (default: adam)
• --model_dir : Path to save model checkpoints
• --model_name : File name used for model checkpoints
• --shuffle_each_epoch : Shuffle training dataset for each epoch (default: True)
• --sort_by_length : Sort pre-fetched minibatches by their target sequence lengths (default: True)

Decoding params

• --beam_width : Beam width used in beamsearch (default: 1)
• --decode_batch_size : Batch size used in decoding
• --max_decode_step : Maximum time step limit in decoding (default: 500)
• --write_n_best : Write beamsearch n-best list (n=beam_width) (default: False)
• --decode_input : Input file path to decode
• --decode_output : Output file path of decoding output

Runtime params

• --allow_soft_placement : Allow device soft placement
• --log_device_placement : Log placement of ops on devices

The implementation is based on following projects:

• nematus: Theano implementation of Neural Machine Translation. Major reference of this project
• subword-nmt: Included subword-unit scripts to preprocess input data
• moses: Included preprocessing scripts to preprocess input data
• tf.seq2seq_legacy Legacy Tensorflow seq2seq tutorial
• tf_tutorial_plus: Nice tutorials for tf.contrib.seq2seq API

For any comments and feedbacks, please email me at [email protected] or open an issue here.