Source code for nltk.tag.crf

# Natural Language Toolkit: Interface to the CRFSuite Tagger
# Copyright (C) 2001-2023 NLTK Project
# Author: Long Duong <>
# URL: <>
# For license information, see LICENSE.TXT

A module for POS tagging using CRFSuite

import re
import unicodedata

from nltk.tag.api import TaggerI

    import pycrfsuite
except ImportError:

[docs]class CRFTagger(TaggerI): """ A module for POS tagging using CRFSuite >>> from nltk.tag import CRFTagger >>> ct = CRFTagger() # doctest: +SKIP >>> train_data = [[('University','Noun'), ('is','Verb'), ('a','Det'), ('good','Adj'), ('place','Noun')], ... [('dog','Noun'),('eat','Verb'),('meat','Noun')]] >>> ct.train(train_data,'model.crf.tagger') # doctest: +SKIP >>> ct.tag_sents([['dog','is','good'], ['Cat','eat','meat']]) # doctest: +SKIP [[('dog', 'Noun'), ('is', 'Verb'), ('good', 'Adj')], [('Cat', 'Noun'), ('eat', 'Verb'), ('meat', 'Noun')]] >>> gold_sentences = [[('dog','Noun'),('is','Verb'),('good','Adj')] , [('Cat','Noun'),('eat','Verb'), ('meat','Noun')]] >>> ct.accuracy(gold_sentences) # doctest: +SKIP 1.0 Setting learned model file >>> ct = CRFTagger() # doctest: +SKIP >>> ct.set_model_file('model.crf.tagger') # doctest: +SKIP >>> ct.accuracy(gold_sentences) # doctest: +SKIP 1.0 """
[docs] def __init__(self, feature_func=None, verbose=False, training_opt={}): """ Initialize the CRFSuite tagger :param feature_func: The function that extracts features for each token of a sentence. This function should take 2 parameters: tokens and index which extract features at index position from tokens list. See the build in _get_features function for more detail. :param verbose: output the debugging messages during training. :type verbose: boolean :param training_opt: python-crfsuite training options :type training_opt: dictionary Set of possible training options (using LBFGS training algorithm). :'feature.minfreq': The minimum frequency of features. :'feature.possible_states': Force to generate possible state features. :'feature.possible_transitions': Force to generate possible transition features. :'c1': Coefficient for L1 regularization. :'c2': Coefficient for L2 regularization. :'max_iterations': The maximum number of iterations for L-BFGS optimization. :'num_memories': The number of limited memories for approximating the inverse hessian matrix. :'epsilon': Epsilon for testing the convergence of the objective. :'period': The duration of iterations to test the stopping criterion. :'delta': The threshold for the stopping criterion; an L-BFGS iteration stops when the improvement of the log likelihood over the last ${period} iterations is no greater than this threshold. :'linesearch': The line search algorithm used in L-BFGS updates: - 'MoreThuente': More and Thuente's method, - 'Backtracking': Backtracking method with regular Wolfe condition, - 'StrongBacktracking': Backtracking method with strong Wolfe condition :'max_linesearch': The maximum number of trials for the line search algorithm. """ self._model_file = "" self._tagger = pycrfsuite.Tagger() if feature_func is None: self._feature_func = self._get_features else: self._feature_func = feature_func self._verbose = verbose self._training_options = training_opt self._pattern = re.compile(r"\d")
[docs] def set_model_file(self, model_file): self._model_file = model_file
def _get_features(self, tokens, idx): """ Extract basic features about this word including - Current word - is it capitalized? - Does it have punctuation? - Does it have a number? - Suffixes up to length 3 Note that : we might include feature over previous word, next word etc. :return: a list which contains the features :rtype: list(str) """ token = tokens[idx] feature_list = [] if not token: return feature_list # Capitalization if token[0].isupper(): feature_list.append("CAPITALIZATION") # Number if, token) is not None: feature_list.append("HAS_NUM") # Punctuation punc_cat = {"Pc", "Pd", "Ps", "Pe", "Pi", "Pf", "Po"} if all(unicodedata.category(x) in punc_cat for x in token): feature_list.append("PUNCTUATION") # Suffix up to length 3 if len(token) > 1: feature_list.append("SUF_" + token[-1:]) if len(token) > 2: feature_list.append("SUF_" + token[-2:]) if len(token) > 3: feature_list.append("SUF_" + token[-3:]) feature_list.append("WORD_" + token) return feature_list
[docs] def tag_sents(self, sents): """ Tag a list of sentences. NB before using this function, user should specify the mode_file either by - Train a new model using ``train`` function - Use the pre-trained model which is set via ``set_model_file`` function :params sentences: list of sentences needed to tag. :type sentences: list(list(str)) :return: list of tagged sentences. :rtype: list(list(tuple(str,str))) """ if self._model_file == "": raise Exception( " No model file is found !! Please use train or set_model_file function" ) # We need the list of sentences instead of the list generator for matching the input and output result = [] for tokens in sents: features = [self._feature_func(tokens, i) for i in range(len(tokens))] labels = self._tagger.tag(features) if len(labels) != len(tokens): raise Exception(" Predicted Length Not Matched, Expect Errors !") tagged_sent = list(zip(tokens, labels)) result.append(tagged_sent) return result
[docs] def train(self, train_data, model_file): """ Train the CRF tagger using CRFSuite :params train_data : is the list of annotated sentences. :type train_data : list (list(tuple(str,str))) :params model_file : the model will be saved to this file. """ trainer = pycrfsuite.Trainer(verbose=self._verbose) trainer.set_params(self._training_options) for sent in train_data: tokens, labels = zip(*sent) features = [self._feature_func(tokens, i) for i in range(len(tokens))] trainer.append(features, labels) # Now train the model, the output should be model_file trainer.train(model_file) # Save the model file self.set_model_file(model_file)
[docs] def tag(self, tokens): """ Tag a sentence using Python CRFSuite Tagger. NB before using this function, user should specify the mode_file either by - Train a new model using ``train`` function - Use the pre-trained model which is set via ``set_model_file`` function :params tokens: list of tokens needed to tag. :type tokens: list(str) :return: list of tagged tokens. :rtype: list(tuple(str,str)) """ return self.tag_sents([tokens])[0]