Step 0: Install OpenNMT-py

pip install --upgrade pip
pip install OpenNMT-py

Step 1: Prepare the data

To get started, we propose to download a toy English-German dataset for machine translation containing 10k tokenized sentences:

wget https://s3.amazonaws.com/opennmt-trainingdata/toy-ende.tar.gz
tar xf toy-ende.tar.gz
cd toy-ende

The data consists of parallel source (src) and target (tgt) data containing one sentence per line with tokens separated by a space:

  • src-train.txt

  • tgt-train.txt

  • src-val.txt

  • tgt-val.txt

Validation files are used to evaluate the convergence of the training. It usually contains no more than 5k sentences.

$ head -n 3 toy_ende/src-train.txt
It is not acceptable that , with the help of the national bureaucracies , Parliament 's legislative prerogative should be made null and void by means of implementing provisions whose content , purpose and extent are not laid down in advance .
Federal Master Trainer and Senior Instructor of the Italian Federation of Aerobic Fitness , Group Fitness , Postural Gym , Stretching and Pilates; from 2004 , he has been collaborating with Antiche Terme as personal Trainer and Instructor of Stretching , Pilates and Postural Gym .
" Two soldiers came up to me and told me that if I refuse to sleep with them , they will kill me . They beat me and ripped my clothes .

We need to build a YAML configuration file to specify the data that will be used:

# toy_en_de.yaml

## Where the samples will be written
save_data: toy-ende/run/example
## Where the vocab(s) will be written
src_vocab: toy-ende/run/example.vocab.src
tgt_vocab: toy-ende/run/example.vocab.tgt
# Prevent overwriting existing files in the folder
overwrite: False

# Corpus opts:
        path_src: toy-ende/src-train.txt
        path_tgt: toy-ende/tgt-train.txt
        path_src: toy-ende/src-val.txt
        path_tgt: toy-ende/tgt-val.txt

From this configuration, we can build the vocab(s), that will be necessary to train the model:

onmt_build_vocab -config toy_en_de.yaml -n_sample 10000


  • -n_sample is required here – it represents the number of lines sampled from each corpus to build the vocab.

  • This configuration is the simplest possible, without any tokenization or other transforms. See other example configurations for more complex pipelines.

Step 2: Train the model

To train a model, we need to add the following to the YAML configuration file:

  • the vocabulary path(s) that will be used: can be that generated by onmt_build_vocab;

  • training specific parameters.

# toy_en_de.yaml


# Vocabulary files that were just created
src_vocab: toy-ende/run/example.vocab.src
tgt_vocab: toy-ende/run/example.vocab.tgt

# Train on a single GPU
world_size: 1
gpu_ranks: [0]

# Where to save the checkpoints
save_model: toy-ende/run/model
save_checkpoint_steps: 500
train_steps: 1000
valid_steps: 500

Then you can simply run:

onmt_train -config toy_en_de.yaml

This configuration will run the default model, which consists of a 2-layer LSTM with 500 hidden units on both the encoder and decoder. It will run on a single GPU (world_size 1 & gpu_ranks [0]).

Before the training process actually starts, the *.vocab.pt together with *.transforms.pt can be dumped to -save_data with configurations specified in -config yaml file by enabling the -dump_fields and -dump_transforms flags. It is also possible to generate transformed samples to simplify any potentially required visual inspection. The number of sample lines to dump per corpus is set with the -n_sample flag.

For more advanded models and parameters, see other example configurations or the FAQ.

Step 3: Translate

onmt_translate -model toy-ende/run/model_step_1000.pt -src toy-ende/src-test.txt -output toy-ende/pred_1000.txt -gpu 0 -verbose

Now you have a model which you can use to predict on new data. We do this by running beam search. This will output predictions into toy-ende/pred_1000.txt.


The predictions are going to be quite terrible, as the demo dataset is small. Try running on some larger datasets! For example you can download millions of parallel sentences for translation or summarization.