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Python bindings for the BLLIP natural language parser

Project description

The BLLIP parser (also known as the Charniak-Johnson parser or Brown Reranking Parser) is described in the paper Charniak and Johnson (Association of Computational Linguistics, 2005). This package provides the BLLIP parser runtime along with a Python interface. Note that it does not come with any parsing models but includes a downloader. The primary maintenance for the parser takes place at GitHub.

Fetching parsing models

Before you can parse, you’ll need some parsing models. ModelFetcher will help you download and install parsing models. It can be invoked from the command line. For example, this will download and install the standard WSJ model:

shell% python -m bllipparser.ModelFetcher -i WSJ

Run python -mbllipparser.ModelFetcher with no arguments for a full listing of options and available parsing models. It can also be invoked as a Python library:

>>> from bllipparser.ModelFetcher import download_and_install_model
>>> download_and_install_model('WSJ', '/tmp/models')

In this case, it would download WSJ and install it to /tmp/models/WSJ. Note that it returns the path to the downloaded model.

Basic usage

The easiest way to construct a parser is with the from_unified_model_dir class method. A unified model is a directory that contains two subdirectories: parser/ and reranker/, each with the respective model files:

>>> from bllipparser import RerankingParser, tokenize
>>> rrp = RerankingParser.from_unified_model_dir('/path/to/model/')

This can be integrated with ModelFetcher (if the model is already installed, download_and_install_model is a no-op):

>>> model_dir = download_and_install_model('WSJ', '/tmp/models')
>>> rrp = RerankingParser.from_unified_model_dir(model_dir)

You can also load parser and reranker models manually:

>>> rrp = RerankingParser()
>>> rrp.load_parser_model('/tmp/models/WSJ/parser')
>>> rrp.load_reranker_model('/tmp/models/WSJ/reranker')

Parsing a single sentence and reading information about the top parse with parse(). The parser produces an n-best list of the n most likely parses of the sentence (default: n=50). Typically you only want the top parse, but the others are available as well:

>>> nbest_list = rrp.parse('This is a sentence.')

Getting information about the top parse:

>>> print repr(nbest_list[0])
ScoredParse('(S1 (S (NP (DT This)) (VP (VBZ is) (NP (DT a) (NN sentence))) (. .)))', parser_score=-29.621201629004183, reranker_score=-7.9273829816098731)
>>> print nbest_list[0].ptb_parse
(S1 (S (NP (DT This)) (VP (VBZ is) (NP (DT a) (NN sentence))) (. .)))
>>> print nbest_list[0].parser_score
>>> print nbest_list[0].reranker_score
>>> print len(nbest_list)

If you have an existing tokenizer, tokenization can also be specified by passing a list of strings:

>>> nbest_list = rrp.parse(['This', 'is', 'a', 'pretokenized', 'sentence', '.'])

The reranker can be disabled by setting rerank=False:

>>> nbest_list = rrp.parse('Parser only!', rerank=False)

You can also parse text with existing POS tags (these act as soft constraints). In this example, token 0 (‘Time’) should have tag VB and token 1 (‘flies’) should have tag NNS:

>>> rrp.parse_tagged(['Time', 'flies'], possible_tags={0 : 'VB', 1 : 'NNS'})[0]
ScoredParse('(S1 (NP (VB Time) (NNS flies)))', parser_score=-53.94938875760073, reranker_score=-15.841407102717749)

You don’t need to specify a tag for all words: Here, token 0 (‘Time’) should have tag VB and token 1 (‘flies’) is unconstrained:

>>> rrp.parse_tagged(['Time', 'flies'], possible_tags={0 : 'VB'})[0]
ScoredParse('(S1 (S (VP (VB Time) (NP (VBZ flies)))))', parser_score=-54.390430751112156, reranker_score=-17.290145080887005)

You can specify multiple tags for each token. When you do this, the tags for a token will be used in decreasing priority. token 0 (‘Time’) should have tag VB, JJ, or NN and token 1 (‘flies’) is unconstrained:

>>> rrp.parse_tagged(['Time', 'flies'], possible_tags={0 : ['VB', 'JJ', 'NN']})[0]
ScoredParse('(S1 (NP (NN Time) (VBZ flies)))', parser_score=-42.82904107213723, reranker_score=-12.865900776775314)

There are many parser options which can be adjusted (though the defaults should work well for most cases) with set_parser_options. This will change the size of the n-best list and pick the defaults for all other options. It returns a dictionary of the current options:

>>> rrp.set_parser_options(nbest=10)
{'language': 'En', 'case_insensitive': False, 'debug': 0, 'small_corpus': True, 'overparsing': 21, 'smooth_pos': 0, 'nbest': 10}
>>> nbest_list = rrp.parse('The list is smaller now.', rerank=False)
>>> len(nbest_list)

Use this if all you want is a tokenizer:

>>> tokenize("Tokenize this sentence, please.")
['Tokenize', 'this', 'sentence', ',', 'please', '.']

Parsing shell

There is an interactive shell which can help visualize a parse:

shell% python -mbllipparser.ParsingShell /path/to/model

Once in the shell, type a sentence to have the parser parse it:

rrp> I saw the astronomer with the telescope.
Tokens: I saw the astronomer with the telescope .

Parser's parse:
(S1 (S (NP (PRP I))
     (VP (VBD saw)
      (NP (NP (DT the) (NN astronomer))
       (PP (IN with) (NP (DT the) (NN telescope)))))
     (. .)))

Reranker's parse: (parser index 2)
(S1 (S (NP (PRP I))
     (VP (VBD saw)
      (NP (DT the) (NN astronomer))
      (PP (IN with) (NP (DT the) (NN telescope))))
     (. .)))

If you have nltk installed, you can use its tree visualization to see the output:

rrp> visual Show me this parse.
Tokens: Show me this parse .

[graphical display of the parse appears]

There is more detailed help inside the shell under the help command.

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