282 lines
11 KiB
Python
282 lines
11 KiB
Python
# Natural Language Toolkit: Interface to BLLIP Parser
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#
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# Author: David McClosky <dmcc@bigasterisk.com>
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#
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# Copyright (C) 2001-2018 NLTK Project
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# URL: <http://nltk.org/>
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# For license information, see LICENSE.TXT
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from __future__ import print_function
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from nltk.parse.api import ParserI
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from nltk.tree import Tree
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"""
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Interface for parsing with BLLIP Parser. Requires the Python
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bllipparser module. BllipParser objects can be constructed with the
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``BllipParser.from_unified_model_dir`` class method or manually using the
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``BllipParser`` constructor. The former is generally easier if you have
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a BLLIP Parser unified model directory -- a basic model can be obtained
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from NLTK's downloader. More unified parsing models can be obtained with
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BLLIP Parser's ModelFetcher (run ``python -m bllipparser.ModelFetcher``
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or see docs for ``bllipparser.ModelFetcher.download_and_install_model``).
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Basic usage::
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# download and install a basic unified parsing model (Wall Street Journal)
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# sudo python -m nltk.downloader bllip_wsj_no_aux
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>>> from nltk.data import find
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>>> model_dir = find('models/bllip_wsj_no_aux').path
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>>> bllip = BllipParser.from_unified_model_dir(model_dir)
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# 1-best parsing
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>>> sentence1 = 'British left waffles on Falklands .'.split()
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>>> top_parse = bllip.parse_one(sentence1)
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>>> print(top_parse)
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(S1
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(S
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(NP (JJ British) (NN left))
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(VP (VBZ waffles) (PP (IN on) (NP (NNP Falklands))))
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(. .)))
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# n-best parsing
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>>> sentence2 = 'Time flies'.split()
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>>> all_parses = bllip.parse_all(sentence2)
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>>> print(len(all_parses))
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50
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>>> print(all_parses[0])
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(S1 (S (NP (NNP Time)) (VP (VBZ flies))))
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# incorporating external tagging constraints (None means unconstrained tag)
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>>> constrained1 = bllip.tagged_parse([('Time', 'VB'), ('flies', 'NNS')])
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>>> print(next(constrained1))
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(S1 (NP (VB Time) (NNS flies)))
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>>> constrained2 = bllip.tagged_parse([('Time', 'NN'), ('flies', None)])
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>>> print(next(constrained2))
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(S1 (NP (NN Time) (VBZ flies)))
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References
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----------
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- Charniak, Eugene. "A maximum-entropy-inspired parser." Proceedings of
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the 1st North American chapter of the Association for Computational
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Linguistics conference. Association for Computational Linguistics,
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2000.
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- Charniak, Eugene, and Mark Johnson. "Coarse-to-fine n-best parsing
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and MaxEnt discriminative reranking." Proceedings of the 43rd Annual
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Meeting on Association for Computational Linguistics. Association
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for Computational Linguistics, 2005.
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Known issues
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------------
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Note that BLLIP Parser is not currently threadsafe. Since this module
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uses a SWIG interface, it is potentially unsafe to create multiple
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``BllipParser`` objects in the same process. BLLIP Parser currently
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has issues with non-ASCII text and will raise an error if given any.
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See http://pypi.python.org/pypi/bllipparser/ for more information
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on BLLIP Parser's Python interface.
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"""
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__all__ = ['BllipParser']
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# this block allows this module to be imported even if bllipparser isn't
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# available
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try:
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from bllipparser import RerankingParser
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from bllipparser.RerankingParser import get_unified_model_parameters
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def _ensure_bllip_import_or_error():
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pass
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except ImportError as ie:
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def _ensure_bllip_import_or_error(ie=ie):
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raise ImportError("Couldn't import bllipparser module: %s" % ie)
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def _ensure_ascii(words):
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try:
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for i, word in enumerate(words):
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word.decode('ascii')
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except UnicodeDecodeError:
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raise ValueError("Token %d (%r) is non-ASCII. BLLIP Parser "
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"currently doesn't support non-ASCII inputs." %
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(i, word))
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def _scored_parse_to_nltk_tree(scored_parse):
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return Tree.fromstring(str(scored_parse.ptb_parse))
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class BllipParser(ParserI):
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"""
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Interface for parsing with BLLIP Parser. BllipParser objects can be
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constructed with the ``BllipParser.from_unified_model_dir`` class
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method or manually using the ``BllipParser`` constructor.
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"""
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def __init__(self, parser_model=None, reranker_features=None,
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reranker_weights=None, parser_options=None,
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reranker_options=None):
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"""
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Load a BLLIP Parser model from scratch. You'll typically want to
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use the ``from_unified_model_dir()`` class method to construct
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this object.
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:param parser_model: Path to parser model directory
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:type parser_model: str
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:param reranker_features: Path the reranker model's features file
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:type reranker_features: str
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:param reranker_weights: Path the reranker model's weights file
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:type reranker_weights: str
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:param parser_options: optional dictionary of parser options, see
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``bllipparser.RerankingParser.RerankingParser.load_parser_options()``
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for more information.
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:type parser_options: dict(str)
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:param reranker_options: optional
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dictionary of reranker options, see
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``bllipparser.RerankingParser.RerankingParser.load_reranker_model()``
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for more information.
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:type reranker_options: dict(str)
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"""
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_ensure_bllip_import_or_error()
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parser_options = parser_options or {}
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reranker_options = reranker_options or {}
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self.rrp = RerankingParser()
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self.rrp.load_parser_model(parser_model, **parser_options)
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if reranker_features and reranker_weights:
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self.rrp.load_reranker_model(features_filename=reranker_features,
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weights_filename=reranker_weights,
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**reranker_options)
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def parse(self, sentence):
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"""
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Use BLLIP Parser to parse a sentence. Takes a sentence as a list
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of words; it will be automatically tagged with this BLLIP Parser
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instance's tagger.
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:return: An iterator that generates parse trees for the sentence
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from most likely to least likely.
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:param sentence: The sentence to be parsed
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:type sentence: list(str)
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:rtype: iter(Tree)
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"""
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_ensure_ascii(sentence)
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nbest_list = self.rrp.parse(sentence)
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for scored_parse in nbest_list:
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yield _scored_parse_to_nltk_tree(scored_parse)
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def tagged_parse(self, word_and_tag_pairs):
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"""
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Use BLLIP to parse a sentence. Takes a sentence as a list of
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(word, tag) tuples; the sentence must have already been tokenized
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and tagged. BLLIP will attempt to use the tags provided but may
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use others if it can't come up with a complete parse subject
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to those constraints. You may also specify a tag as ``None``
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to leave a token's tag unconstrained.
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:return: An iterator that generates parse trees for the sentence
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from most likely to least likely.
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:param sentence: Input sentence to parse as (word, tag) pairs
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:type sentence: list(tuple(str, str))
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:rtype: iter(Tree)
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"""
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words = []
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tag_map = {}
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for i, (word, tag) in enumerate(word_and_tag_pairs):
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words.append(word)
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if tag is not None:
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tag_map[i] = tag
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_ensure_ascii(words)
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nbest_list = self.rrp.parse_tagged(words, tag_map)
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for scored_parse in nbest_list:
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yield _scored_parse_to_nltk_tree(scored_parse)
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@classmethod
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def from_unified_model_dir(this_class, model_dir, parser_options=None,
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reranker_options=None):
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"""
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Create a ``BllipParser`` object from a unified parsing model
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directory. Unified parsing model directories are a standardized
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way of storing BLLIP parser and reranker models together on disk.
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See ``bllipparser.RerankingParser.get_unified_model_parameters()``
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for more information about unified model directories.
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:return: A ``BllipParser`` object using the parser and reranker
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models in the model directory.
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:param model_dir: Path to the unified model directory.
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:type model_dir: str
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:param parser_options: optional dictionary of parser options, see
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``bllipparser.RerankingParser.RerankingParser.load_parser_options()``
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for more information.
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:type parser_options: dict(str)
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:param reranker_options: optional dictionary of reranker options, see
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``bllipparser.RerankingParser.RerankingParser.load_reranker_model()``
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for more information.
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:type reranker_options: dict(str)
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:rtype: BllipParser
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"""
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(parser_model_dir, reranker_features_filename,
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reranker_weights_filename) = get_unified_model_parameters(model_dir)
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return this_class(parser_model_dir, reranker_features_filename,
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reranker_weights_filename, parser_options,
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reranker_options)
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def demo():
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"""This assumes the Python module bllipparser is installed."""
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# download and install a basic unified parsing model (Wall Street Journal)
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# sudo python -m nltk.downloader bllip_wsj_no_aux
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from nltk.data import find
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model_dir = find('models/bllip_wsj_no_aux').path
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print('Loading BLLIP Parsing models...')
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# the easiest way to get started is to use a unified model
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bllip = BllipParser.from_unified_model_dir(model_dir)
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print('Done.')
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sentence1 = 'British left waffles on Falklands .'.split()
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sentence2 = 'I saw the man with the telescope .'.split()
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# this sentence is known to fail under the WSJ parsing model
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fail1 = '# ! ? : -'.split()
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for sentence in (sentence1, sentence2, fail1):
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print('Sentence: %r' % ' '.join(sentence))
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try:
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tree = next(bllip.parse(sentence))
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print(tree)
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except StopIteration:
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print("(parse failed)")
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# n-best parsing demo
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for i, parse in enumerate(bllip.parse(sentence1)):
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print('parse %d:\n%s' % (i, parse))
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# using external POS tag constraints
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print("forcing 'tree' to be 'NN':",
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next(bllip.tagged_parse([('A', None), ('tree', 'NN')])))
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print("forcing 'A' to be 'DT' and 'tree' to be 'NNP':",
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next(bllip.tagged_parse([('A', 'DT'), ('tree', 'NNP')])))
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# constraints don't have to make sense... (though on more complicated
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# sentences, they may cause the parse to fail)
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print("forcing 'A' to be 'NNP':",
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next(bllip.tagged_parse([('A', 'NNP'), ('tree', None)])))
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def setup_module(module):
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from nose import SkipTest
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try:
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_ensure_bllip_import_or_error()
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except ImportError:
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raise SkipTest('doctests from nltk.parse.bllip are skipped because '
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'the bllipparser module is not installed')
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