python - training data format for NLTK punkt

Question

I would like to run nltk Punkt to split sentences. There is no training model so I train model separately, but I am not sure if the training data format I am using is correct.

My training data is one sentence per line. I wasn't able to find any documentation about this, only this thread (https://groups.google.com/forum/#!topic/nltk-users/bxIEnmgeCSM) sheds some light about training data format.

What is the correct training data format for NLTK Punkt sentence tokenizer?

Answer 1

Ah yes, Punkt tokenizer is the magical unsupervised sentence boundary detection. And the author's last name is pretty cool too, Kiss and Strunk (2006). The idea is to use NO annotation to train a sentence boundary detector, hence the input will be ANY sort of plaintext (as long as the encoding is consistent).

To train a new model, simply use:

import nltk.tokenize.punkt
import pickle
import codecs
tokenizer = nltk.tokenize.punkt.PunktSentenceTokenizer()
text = codecs.open("someplain.txt","r","utf8").read()
tokenizer.train(text)
out = open("someplain.pk","wb")
pickle.dump(tokenizer, out)
out.close()

To achieve higher precision and allow you to stop training at any time and still save a proper pickle for your tokenizer, do look at this code snippet for training a German sentence tokenizer, https://github.com/alvations/DLTK/blob/master/dltk/tokenize/tokenizer.py :

def train_punktsent(trainfile, modelfile):
  """ Trains an unsupervised NLTK punkt sentence tokenizer. """
  punkt = PunktTrainer()
  try:
    with codecs.open(trainfile, 'r','utf8') as fin:
      punkt.train(fin.read(), finalize=False, verbose=False)
  except KeyboardInterrupt:
    print 'KeyboardInterrupt: Stopping the reading of the dump early!'
  ##HACK: Adds abbreviations from rb_tokenizer.
  abbrv_sent = " ".join([i.strip() for i in 
                         codecs.open('abbrev.lex','r','utf8').readlines()])
  abbrv_sent = "Start"+abbrv_sent+"End."
  punkt.train(abbrv_sent,finalize=False, verbose=False)
  # Finalize and outputs trained model.
  punkt.finalize_training(verbose=True)
  model = PunktSentenceTokenizer(punkt.get_params())
  with open(modelfile, mode='wb') as fout:
    pickle.dump(model, fout, protocol=pickle.HIGHEST_PROTOCOL)
  return model

However do note that the period detection is very sensitive to the latin fullstop, question mark and exclamation mark. If you're going to train a punkt tokenizer for other languages that doesn't use latin orthography, you'll need to somehow hack the code to use the appropriate sentence boundary punctuation. If you're using NLTK's implementation of punkt, edit the sent_end_chars variable.

There are pre-trained models available other than the 'default' English tokenizer using nltk.tokenize.sent_tokenize(). Here they are: https://github.com/evandrix/nltk_data/tree/master/tokenizers/punkt

Edited

Note the pre-trained models are currently not available because the nltk_data github repo listed above has been removed.