Open Source Neural Machine Translation in PyTorch
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OpenNMT-py: Open-Source Neural Machine Translation

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This is a Pytorch port of OpenNMT, an open-source (MIT) neural machine translation system. It is designed to be research friendly to try out new ideas in translation, summary, image-to-text, morphology, and many other domains.

Codebase is relatively stable, but PyTorch is still evolving. We currently recommend forking if you need to have stable code.

OpenNMT-py is run as a collaborative open-source project. It is maintained by Sasha Rush (Cambridge, MA), Ben Peters (Saarbrücken), and Jianyu Zhan (Shenzhen). The original code was written by Adam Lerer (NYC). We love contributions. Please consult the Issues page for any Contributions Welcome tagged post.

Table of Contents


pip install -r requirements.txt

You might also need to install torch and torchtext

pip install torch torchtext


The following OpenNMT features are implemented:

Beta Features (committed):

  • multi-GPU
  • Structured attention
  • [Conv2Conv convolution model]
  • SRU "RNNs faster than CNN" paper


Full Documentation

Step 1: Preprocess the data

python -train_src data/src-train.txt -train_tgt data/tgt-train.txt -valid_src data/src-val.txt -valid_tgt data/tgt-val.txt -save_data data/demo

We will be working with some example data in data/ folder.

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 required and used to evaluate the convergence of the training. It usually contains no more than 5000 sentences.

After running the preprocessing, the following files are generated:

  • serialized PyTorch file containing training data
  • serialized PyTorch file containing validation data
  • serialized PyTorch file containing vocabulary data

Internally the system never touches the words themselves, but uses these indices.

Step 2: Train the model

python -data data/demo -save_model demo-model

The main train command is quite simple. Minimally it takes a data file and a save file. This will run the default model, which consists of a 2-layer LSTM with 500 hidden units on both the encoder/decoder. You can also add -gpuid 1 to use (say) GPU 1.

Step 3: Translate

python -model -src data/src-test.txt -output pred.txt -replace_unk -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 pred.txt.

!!! note "Note" 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.

Pretrained embeddings (e.g. GloVe)

Go to tutorial: How to use GloVe pre-trained embeddings in OpenNMT-py

Pretrained Models

The following pretrained models can be downloaded and used with


OpenNMT technical report

  author    = {Guillaume Klein and
               Yoon Kim and
               Yuntian Deng and
               Jean Senellart and
               Alexander M. Rush},
  title     = {OpenNMT: Open-Source Toolkit for Neural Machine Translation},
  booktitle = {Proc. ACL},
  year      = {2017},
  url       = {},
  doi       = {10.18653/v1/P17-4012}