lpngram

Python library for ngram collection and frequency smoothing.

lpngram is a pure-Python implementation of methods for ngram collection and frequency smoothing, originally part of the lingpy library. It has no dependencies, but will use numpy and scipy, if available, to speed smoothing computations. It was designed to work on any kind of sequence, not just words, and has been successfully used to collect phoneme n-grams.

Changelog

Version 0.1:

• First public release.

Installation

In any standard Python environment, lpngram can be installed with:

pip install lpngram

The pip installation will also fetch the dependencies numpy and scipy. If those are not desired, the library can be used by just copying the files in the lpngram directory.

How to use

The library operates on any kind of Python iterable, such as strings, lists, and tuples. There are methods to collect normal n-grams, skip n-grams, and positional n-grams. Different left and right orders can be specified, as well as different padding symbols (if any).

The example below collects ngrams with a left order of at most 1 and a right order of at most 2 from a short list with three country names.

>>> import lpngram
>>> words = ['Germany', 'Italy', 'Brazil']
>>> model = lpngram.NgramModel(1, 2, sequences=words)

Even without smoothing, the model allows you to query counters for specific contexts. Here we investigate which characters are found preceding an a, which are found between G and r, and the full list of characters with their counts:

>>> model._ngrams['###', 'a']
Counter({'m': 1, 't': 1, 'r': 1})
>>> model._ngrams['G', '###', 'r']
Counter({'e': 1})
>>> model._ngrams['###',]
Counter({'a': 3, 'r': 2, 'y': 2, 'l': 2, 'G': 1, 'e': 1, 'm': 1, 'n': 1,
'I': 1, 't': 1, 'B': 1, 'z': 1, 'i': 1})

For most operations, smoothing is necessary or recommended. The library includes a range of smoothing methods, including one based on degree of certainty developed for linguistic investigation purposes.

Here we perform smoothing with Lidstone's method, a gamma of 0.1, and no normalization:

>>> model.train(method='lidstone', gamma=0.1)
>>> model._p['###', 'a']
{'m': -1.363304842895192, 't': -1.363304842895192, 'r': -1.363304842895192}
>>> model._p['G', '###', 'r']
{'e': -0.737598943130779}
>>> model._p['###',]
{'G': -2.864794916106515, 'e': -2.864794916106515, 'r': -2.2181677511814626,
'm': -2.864794916106515, 'a': -1.8287029844197393, 'n': -2.864794916106515,
'y': -2.2181677511814626, 'I': -2.864794916106515, 't': -2.864794916106515,
'l': -2.2181677511814626, 'B': -2.864794916106515, 'z': -2.864794916106515,
'i': -2.864794916106515}

The smoothed distribution allows us to perform the main purpose of the library, which is to score the likelihood of sequences:

>>> model.score("Italy")
-35.461238155043674
>>> [model.score(word) for word in ["Italy", "Itazily", "France"]]
[-35.461238155043674, -106.65033225683297, -240.5559013433157]

We can also compute the internal measures of entropy and perplexity:

>>> model.model_entropy()
62.59647855466861
>>> model.entropy('Itazil')
17.095797405180004
>>> model.perplexity('Itazil')
140070.86762308443
>>> [model.entropy(word) for word in ["Italy", "Itazily", "France"]]
[10.231950486012801, 21.980557922299024, 57.84146765409605]
>>> [model.perplexity(word) for word in ["Italy", "Itazily", "France"]]
[1202.6077837373584, 4138159.7865280183, 2.5823598282235027e+17]

With a smoothed distribution, we can use other methods such as generation of random strings:

>>> model.random_seqs(k=4)
[('B', 'r', 'a', 'z', 'i', 'l'), ('I', 't', 'a', 'z', 'i', 'l'),
('G', 'e', 'r', 'm', 'a', 'n', 'y'), ('I', 't', 'a', 'z', 'i', 'l', 'y')]

Detailed usage is demonstrated in the tests suite. Full documentation and examples will be provided in future versions.

Community guidelines

Contributing guidelines can be found in the CONTRIBUTING.md file.

Authors and citation

The library is developed by Tiago Tresoldi ([email protected]) and Johann-Mattis List ([email protected]).

The authors have received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. ERC Grant #715618, Computer-Assisted Language Comparison.

If you use lpngram, please cite it as:

Tresoldi, Tiago; List, Johann-Mattis (2020). lpngram, a Python library for ngram collection and frequency smoothing. Version 0.1.1. Jena: Max Planck Institute for the Science of Human History. Available at: https://github.com/lingpy/lpngram

In BibTeX:

@misc{Tresoldi2020lpngram,
  author = {Tresoldi, Tiago; List, Johann-Mattis},
  title = {lpngram, a Python library for ngram collection and frequency smoothing. Version 0.1.1},
  howpublished = {\url{https://github.com/lingpy/lpngram}},
  address = {Jena},
  publisher = {Max Planck Institute for the Science of Human History},
  year = {2020},
}