Steps to convert SentencePiece vocab to OpenNMT-py vocab

If we are going to build vocabulary from scratch for OpenNMT-py, we should use something like this:

# -config: path to your config.yaml file
# -n_sample: use -1 to build vocabulary on all the segment in the training dataset
# -num_threads: change it to match the number of CPUs to run it faster

onmt_build_vocab -config config.yaml -n_sample -1 -num_threads 2

However, many of us use SentencePiece for sub-wording, which generates both a sub-wording model and a vocabulary list. We cannot use this vocab file generated by SentencePiece directly in OpenNMT-py. So, if we want to, we have to convert it to a version compatible with OpenNMT-py. Note that the new vocab file will be 3 lines less as the script removes the default tokens in OpenNMT-py, e.g. <unk>, <s>, and </s>.

git clone
cd OpenNMT-py
python3 install
cd ..
cat spm.vocab | python3 OpenNMT-py/tools/ > spm.onmt_vocab

Kind regards,