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710 lines
27 KiB
Python
710 lines
27 KiB
Python
"""
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Porter Stemmer
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This is the Porter stemming algorithm. It follows the algorithm
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presented in
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Porter, M. "An algorithm for suffix stripping." Program 14.3 (1980): 130-137.
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with some optional deviations that can be turned on or off with the
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`mode` argument to the constructor.
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Martin Porter, the algorithm's inventor, maintains a web page about the
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algorithm at
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http://www.tartarus.org/~martin/PorterStemmer/
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which includes another Python implementation and other implementations
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in many languages.
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"""
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__docformat__ = "plaintext"
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import re
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from nltk.stem.api import StemmerI
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class PorterStemmer(StemmerI):
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"""
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A word stemmer based on the Porter stemming algorithm.
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Porter, M. "An algorithm for suffix stripping."
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Program 14.3 (1980): 130-137.
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See http://www.tartarus.org/~martin/PorterStemmer/ for the homepage
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of the algorithm.
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Martin Porter has endorsed several modifications to the Porter
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algorithm since writing his original paper, and those extensions are
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included in the implementations on his website. Additionally, others
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have proposed further improvements to the algorithm, including NLTK
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contributors. There are thus three modes that can be selected by
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passing the appropriate constant to the class constructor's `mode`
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attribute:
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PorterStemmer.ORIGINAL_ALGORITHM
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- Implementation that is faithful to the original paper.
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Note that Martin Porter has deprecated this version of the
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algorithm. Martin distributes implementations of the Porter
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Stemmer in many languages, hosted at:
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http://www.tartarus.org/~martin/PorterStemmer/
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and all of these implementations include his extensions. He
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strongly recommends against using the original, published
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version of the algorithm; only use this mode if you clearly
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understand why you are choosing to do so.
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PorterStemmer.MARTIN_EXTENSIONS
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- Implementation that only uses the modifications to the
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algorithm that are included in the implementations on Martin
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Porter's website. He has declared Porter frozen, so the
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behaviour of those implementations should never change.
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PorterStemmer.NLTK_EXTENSIONS (default)
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- Implementation that includes further improvements devised by
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NLTK contributors or taken from other modified implementations
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found on the web.
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For the best stemming, you should use the default NLTK_EXTENSIONS
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version. However, if you need to get the same results as either the
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original algorithm or one of Martin Porter's hosted versions for
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compatibility with an existing implementation or dataset, you can use
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one of the other modes instead.
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"""
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# Modes the Stemmer can be instantiated in
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NLTK_EXTENSIONS = "NLTK_EXTENSIONS"
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MARTIN_EXTENSIONS = "MARTIN_EXTENSIONS"
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ORIGINAL_ALGORITHM = "ORIGINAL_ALGORITHM"
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def __init__(self, mode=NLTK_EXTENSIONS):
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if mode not in (
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self.NLTK_EXTENSIONS,
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self.MARTIN_EXTENSIONS,
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self.ORIGINAL_ALGORITHM,
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):
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raise ValueError(
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"Mode must be one of PorterStemmer.NLTK_EXTENSIONS, "
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"PorterStemmer.MARTIN_EXTENSIONS, or "
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"PorterStemmer.ORIGINAL_ALGORITHM"
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)
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self.mode = mode
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if self.mode == self.NLTK_EXTENSIONS:
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# This is a table of irregular forms. It is quite short,
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# but still reflects the errors actually drawn to Martin
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# Porter's attention over a 20 year period!
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irregular_forms = {
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"sky": ["sky", "skies"],
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"die": ["dying"],
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"lie": ["lying"],
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"tie": ["tying"],
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"news": ["news"],
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"inning": ["innings", "inning"],
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"outing": ["outings", "outing"],
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"canning": ["cannings", "canning"],
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"howe": ["howe"],
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"proceed": ["proceed"],
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"exceed": ["exceed"],
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"succeed": ["succeed"],
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}
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self.pool = {}
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for key in irregular_forms:
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for val in irregular_forms[key]:
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self.pool[val] = key
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self.vowels = frozenset(["a", "e", "i", "o", "u"])
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def _is_consonant(self, word, i):
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"""Returns True if word[i] is a consonant, False otherwise
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A consonant is defined in the paper as follows:
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A consonant in a word is a letter other than A, E, I, O or
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U, and other than Y preceded by a consonant. (The fact that
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the term `consonant' is defined to some extent in terms of
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itself does not make it ambiguous.) So in TOY the consonants
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are T and Y, and in SYZYGY they are S, Z and G. If a letter
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is not a consonant it is a vowel.
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"""
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if word[i] in self.vowels:
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return False
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if word[i] == "y":
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if i == 0:
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return True
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else:
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return not self._is_consonant(word, i - 1)
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return True
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def _measure(self, stem):
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"""Returns the 'measure' of stem, per definition in the paper
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From the paper:
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A consonant will be denoted by c, a vowel by v. A list
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ccc... of length greater than 0 will be denoted by C, and a
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list vvv... of length greater than 0 will be denoted by V.
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Any word, or part of a word, therefore has one of the four
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forms:
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CVCV ... C
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CVCV ... V
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VCVC ... C
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VCVC ... V
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These may all be represented by the single form
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[C]VCVC ... [V]
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where the square brackets denote arbitrary presence of their
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contents. Using (VC){m} to denote VC repeated m times, this
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may again be written as
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[C](VC){m}[V].
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m will be called the \measure\ of any word or word part when
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represented in this form. The case m = 0 covers the null
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word. Here are some examples:
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m=0 TR, EE, TREE, Y, BY.
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m=1 TROUBLE, OATS, TREES, IVY.
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m=2 TROUBLES, PRIVATE, OATEN, ORRERY.
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"""
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cv_sequence = ""
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# Construct a string of 'c's and 'v's representing whether each
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# character in `stem` is a consonant or a vowel.
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# e.g. 'falafel' becomes 'cvcvcvc',
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# 'architecture' becomes 'vcccvcvccvcv'
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for i in range(len(stem)):
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if self._is_consonant(stem, i):
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cv_sequence += "c"
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else:
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cv_sequence += "v"
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# Count the number of 'vc' occurences, which is equivalent to
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# the number of 'VC' occurrences in Porter's reduced form in the
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# docstring above, which is in turn equivalent to `m`
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return cv_sequence.count("vc")
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def _has_positive_measure(self, stem):
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return self._measure(stem) > 0
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def _contains_vowel(self, stem):
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"""Returns True if stem contains a vowel, else False"""
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for i in range(len(stem)):
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if not self._is_consonant(stem, i):
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return True
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return False
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def _ends_double_consonant(self, word):
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"""Implements condition *d from the paper
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Returns True if word ends with a double consonant
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"""
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return (
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len(word) >= 2
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and word[-1] == word[-2]
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and self._is_consonant(word, len(word) - 1)
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)
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def _ends_cvc(self, word):
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"""Implements condition *o from the paper
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From the paper:
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*o - the stem ends cvc, where the second c is not W, X or Y
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(e.g. -WIL, -HOP).
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"""
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return (
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len(word) >= 3
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and self._is_consonant(word, len(word) - 3)
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and not self._is_consonant(word, len(word) - 2)
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and self._is_consonant(word, len(word) - 1)
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and word[-1] not in ("w", "x", "y")
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) or (
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self.mode == self.NLTK_EXTENSIONS
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and len(word) == 2
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and not self._is_consonant(word, 0)
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and self._is_consonant(word, 1)
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)
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def _replace_suffix(self, word, suffix, replacement):
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"""Replaces `suffix` of `word` with `replacement"""
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assert word.endswith(suffix), "Given word doesn't end with given suffix"
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if suffix == "":
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return word + replacement
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else:
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return word[: -len(suffix)] + replacement
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def _apply_rule_list(self, word, rules):
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"""Applies the first applicable suffix-removal rule to the word
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Takes a word and a list of suffix-removal rules represented as
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3-tuples, with the first element being the suffix to remove,
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the second element being the string to replace it with, and the
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final element being the condition for the rule to be applicable,
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or None if the rule is unconditional.
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"""
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for rule in rules:
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suffix, replacement, condition = rule
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if suffix == "*d" and self._ends_double_consonant(word):
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stem = word[:-2]
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if condition is None or condition(stem):
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return stem + replacement
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else:
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# Don't try any further rules
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return word
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if word.endswith(suffix):
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stem = self._replace_suffix(word, suffix, "")
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if condition is None or condition(stem):
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return stem + replacement
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else:
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# Don't try any further rules
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return word
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return word
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def _step1a(self, word):
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"""Implements Step 1a from "An algorithm for suffix stripping"
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From the paper:
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SSES -> SS caresses -> caress
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IES -> I ponies -> poni
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ties -> ti
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SS -> SS caress -> caress
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S -> cats -> cat
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"""
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# this NLTK-only rule extends the original algorithm, so
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# that 'flies'->'fli' but 'dies'->'die' etc
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if self.mode == self.NLTK_EXTENSIONS:
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if word.endswith("ies") and len(word) == 4:
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return self._replace_suffix(word, "ies", "ie")
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return self._apply_rule_list(
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word,
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[
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("sses", "ss", None), # SSES -> SS
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("ies", "i", None), # IES -> I
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("ss", "ss", None), # SS -> SS
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("s", "", None), # S ->
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],
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)
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def _step1b(self, word):
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"""Implements Step 1b from "An algorithm for suffix stripping"
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From the paper:
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(m>0) EED -> EE feed -> feed
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agreed -> agree
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(*v*) ED -> plastered -> plaster
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bled -> bled
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(*v*) ING -> motoring -> motor
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sing -> sing
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If the second or third of the rules in Step 1b is successful,
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the following is done:
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AT -> ATE conflat(ed) -> conflate
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BL -> BLE troubl(ed) -> trouble
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IZ -> IZE siz(ed) -> size
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(*d and not (*L or *S or *Z))
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-> single letter
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hopp(ing) -> hop
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tann(ed) -> tan
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fall(ing) -> fall
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hiss(ing) -> hiss
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fizz(ed) -> fizz
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(m=1 and *o) -> E fail(ing) -> fail
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fil(ing) -> file
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The rule to map to a single letter causes the removal of one of
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the double letter pair. The -E is put back on -AT, -BL and -IZ,
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so that the suffixes -ATE, -BLE and -IZE can be recognised
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later. This E may be removed in step 4.
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"""
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# this NLTK-only block extends the original algorithm, so that
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# 'spied'->'spi' but 'died'->'die' etc
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if self.mode == self.NLTK_EXTENSIONS:
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if word.endswith("ied"):
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if len(word) == 4:
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return self._replace_suffix(word, "ied", "ie")
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else:
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return self._replace_suffix(word, "ied", "i")
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# (m>0) EED -> EE
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if word.endswith("eed"):
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stem = self._replace_suffix(word, "eed", "")
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if self._measure(stem) > 0:
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return stem + "ee"
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else:
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return word
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rule_2_or_3_succeeded = False
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for suffix in ["ed", "ing"]:
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if word.endswith(suffix):
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intermediate_stem = self._replace_suffix(word, suffix, "")
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if self._contains_vowel(intermediate_stem):
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rule_2_or_3_succeeded = True
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break
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if not rule_2_or_3_succeeded:
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return word
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return self._apply_rule_list(
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intermediate_stem,
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[
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("at", "ate", None), # AT -> ATE
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("bl", "ble", None), # BL -> BLE
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("iz", "ize", None), # IZ -> IZE
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# (*d and not (*L or *S or *Z))
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# -> single letter
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(
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"*d",
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intermediate_stem[-1],
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lambda stem: intermediate_stem[-1] not in ("l", "s", "z"),
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),
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# (m=1 and *o) -> E
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(
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"",
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"e",
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lambda stem: (self._measure(stem) == 1 and self._ends_cvc(stem)),
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),
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],
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)
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def _step1c(self, word):
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"""Implements Step 1c from "An algorithm for suffix stripping"
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From the paper:
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Step 1c
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(*v*) Y -> I happy -> happi
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sky -> sky
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"""
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def nltk_condition(stem):
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"""
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This has been modified from the original Porter algorithm so
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that y->i is only done when y is preceded by a consonant,
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but not if the stem is only a single consonant, i.e.
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(*c and not c) Y -> I
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So 'happy' -> 'happi', but
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'enjoy' -> 'enjoy' etc
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This is a much better rule. Formerly 'enjoy'->'enjoi' and
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'enjoyment'->'enjoy'. Step 1c is perhaps done too soon; but
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with this modification that no longer really matters.
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Also, the removal of the contains_vowel(z) condition means
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that 'spy', 'fly', 'try' ... stem to 'spi', 'fli', 'tri' and
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conflate with 'spied', 'tried', 'flies' ...
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"""
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return len(stem) > 1 and self._is_consonant(stem, len(stem) - 1)
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def original_condition(stem):
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return self._contains_vowel(stem)
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return self._apply_rule_list(
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word,
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[
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(
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"y",
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"i",
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nltk_condition
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if self.mode == self.NLTK_EXTENSIONS
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else original_condition,
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)
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],
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)
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def _step2(self, word):
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"""Implements Step 2 from "An algorithm for suffix stripping"
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From the paper:
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Step 2
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(m>0) ATIONAL -> ATE relational -> relate
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(m>0) TIONAL -> TION conditional -> condition
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rational -> rational
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(m>0) ENCI -> ENCE valenci -> valence
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(m>0) ANCI -> ANCE hesitanci -> hesitance
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(m>0) IZER -> IZE digitizer -> digitize
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(m>0) ABLI -> ABLE conformabli -> conformable
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(m>0) ALLI -> AL radicalli -> radical
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(m>0) ENTLI -> ENT differentli -> different
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(m>0) ELI -> E vileli - > vile
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(m>0) OUSLI -> OUS analogousli -> analogous
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(m>0) IZATION -> IZE vietnamization -> vietnamize
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(m>0) ATION -> ATE predication -> predicate
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(m>0) ATOR -> ATE operator -> operate
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(m>0) ALISM -> AL feudalism -> feudal
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(m>0) IVENESS -> IVE decisiveness -> decisive
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(m>0) FULNESS -> FUL hopefulness -> hopeful
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(m>0) OUSNESS -> OUS callousness -> callous
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(m>0) ALITI -> AL formaliti -> formal
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(m>0) IVITI -> IVE sensitiviti -> sensitive
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(m>0) BILITI -> BLE sensibiliti -> sensible
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"""
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if self.mode == self.NLTK_EXTENSIONS:
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# Instead of applying the ALLI -> AL rule after '(a)bli' per
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# the published algorithm, instead we apply it first, and,
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# if it succeeds, run the result through step2 again.
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if word.endswith("alli") and self._has_positive_measure(
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self._replace_suffix(word, "alli", "")
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):
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return self._step2(self._replace_suffix(word, "alli", "al"))
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bli_rule = ("bli", "ble", self._has_positive_measure)
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abli_rule = ("abli", "able", self._has_positive_measure)
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rules = [
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("ational", "ate", self._has_positive_measure),
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("tional", "tion", self._has_positive_measure),
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("enci", "ence", self._has_positive_measure),
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("anci", "ance", self._has_positive_measure),
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("izer", "ize", self._has_positive_measure),
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abli_rule if self.mode == self.ORIGINAL_ALGORITHM else bli_rule,
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("alli", "al", self._has_positive_measure),
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("entli", "ent", self._has_positive_measure),
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("eli", "e", self._has_positive_measure),
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("ousli", "ous", self._has_positive_measure),
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("ization", "ize", self._has_positive_measure),
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("ation", "ate", self._has_positive_measure),
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("ator", "ate", self._has_positive_measure),
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("alism", "al", self._has_positive_measure),
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("iveness", "ive", self._has_positive_measure),
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("fulness", "ful", self._has_positive_measure),
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("ousness", "ous", self._has_positive_measure),
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("aliti", "al", self._has_positive_measure),
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("iviti", "ive", self._has_positive_measure),
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("biliti", "ble", self._has_positive_measure),
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]
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if self.mode == self.NLTK_EXTENSIONS:
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rules.append(("fulli", "ful", self._has_positive_measure))
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# The 'l' of the 'logi' -> 'log' rule is put with the stem,
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# so that short stems like 'geo' 'theo' etc work like
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# 'archaeo' 'philo' etc.
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rules.append(
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("logi", "log", lambda stem: self._has_positive_measure(word[:-3]))
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)
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if self.mode == self.MARTIN_EXTENSIONS:
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rules.append(("logi", "log", self._has_positive_measure))
|
|
|
|
return self._apply_rule_list(word, rules)
|
|
|
|
def _step3(self, word):
|
|
"""Implements Step 3 from "An algorithm for suffix stripping"
|
|
|
|
From the paper:
|
|
|
|
Step 3
|
|
|
|
(m>0) ICATE -> IC triplicate -> triplic
|
|
(m>0) ATIVE -> formative -> form
|
|
(m>0) ALIZE -> AL formalize -> formal
|
|
(m>0) ICITI -> IC electriciti -> electric
|
|
(m>0) ICAL -> IC electrical -> electric
|
|
(m>0) FUL -> hopeful -> hope
|
|
(m>0) NESS -> goodness -> good
|
|
"""
|
|
return self._apply_rule_list(
|
|
word,
|
|
[
|
|
("icate", "ic", self._has_positive_measure),
|
|
("ative", "", self._has_positive_measure),
|
|
("alize", "al", self._has_positive_measure),
|
|
("iciti", "ic", self._has_positive_measure),
|
|
("ical", "ic", self._has_positive_measure),
|
|
("ful", "", self._has_positive_measure),
|
|
("ness", "", self._has_positive_measure),
|
|
],
|
|
)
|
|
|
|
def _step4(self, word):
|
|
"""Implements Step 4 from "An algorithm for suffix stripping"
|
|
|
|
Step 4
|
|
|
|
(m>1) AL -> revival -> reviv
|
|
(m>1) ANCE -> allowance -> allow
|
|
(m>1) ENCE -> inference -> infer
|
|
(m>1) ER -> airliner -> airlin
|
|
(m>1) IC -> gyroscopic -> gyroscop
|
|
(m>1) ABLE -> adjustable -> adjust
|
|
(m>1) IBLE -> defensible -> defens
|
|
(m>1) ANT -> irritant -> irrit
|
|
(m>1) EMENT -> replacement -> replac
|
|
(m>1) MENT -> adjustment -> adjust
|
|
(m>1) ENT -> dependent -> depend
|
|
(m>1 and (*S or *T)) ION -> adoption -> adopt
|
|
(m>1) OU -> homologou -> homolog
|
|
(m>1) ISM -> communism -> commun
|
|
(m>1) ATE -> activate -> activ
|
|
(m>1) ITI -> angulariti -> angular
|
|
(m>1) OUS -> homologous -> homolog
|
|
(m>1) IVE -> effective -> effect
|
|
(m>1) IZE -> bowdlerize -> bowdler
|
|
|
|
The suffixes are now removed. All that remains is a little
|
|
tidying up.
|
|
"""
|
|
measure_gt_1 = lambda stem: self._measure(stem) > 1
|
|
|
|
return self._apply_rule_list(
|
|
word,
|
|
[
|
|
("al", "", measure_gt_1),
|
|
("ance", "", measure_gt_1),
|
|
("ence", "", measure_gt_1),
|
|
("er", "", measure_gt_1),
|
|
("ic", "", measure_gt_1),
|
|
("able", "", measure_gt_1),
|
|
("ible", "", measure_gt_1),
|
|
("ant", "", measure_gt_1),
|
|
("ement", "", measure_gt_1),
|
|
("ment", "", measure_gt_1),
|
|
("ent", "", measure_gt_1),
|
|
# (m>1 and (*S or *T)) ION ->
|
|
(
|
|
"ion",
|
|
"",
|
|
lambda stem: self._measure(stem) > 1 and stem[-1] in ("s", "t"),
|
|
),
|
|
("ou", "", measure_gt_1),
|
|
("ism", "", measure_gt_1),
|
|
("ate", "", measure_gt_1),
|
|
("iti", "", measure_gt_1),
|
|
("ous", "", measure_gt_1),
|
|
("ive", "", measure_gt_1),
|
|
("ize", "", measure_gt_1),
|
|
],
|
|
)
|
|
|
|
def _step5a(self, word):
|
|
"""Implements Step 5a from "An algorithm for suffix stripping"
|
|
|
|
From the paper:
|
|
|
|
Step 5a
|
|
|
|
(m>1) E -> probate -> probat
|
|
rate -> rate
|
|
(m=1 and not *o) E -> cease -> ceas
|
|
"""
|
|
# Note that Martin's test vocabulary and reference
|
|
# implementations are inconsistent in how they handle the case
|
|
# where two rules both refer to a suffix that matches the word
|
|
# to be stemmed, but only the condition of the second one is
|
|
# true.
|
|
# Earlier in step2b we had the rules:
|
|
# (m>0) EED -> EE
|
|
# (*v*) ED ->
|
|
# but the examples in the paper included "feed"->"feed", even
|
|
# though (*v*) is true for "fe" and therefore the second rule
|
|
# alone would map "feed"->"fe".
|
|
# However, in THIS case, we need to handle the consecutive rules
|
|
# differently and try both conditions (obviously; the second
|
|
# rule here would be redundant otherwise). Martin's paper makes
|
|
# no explicit mention of the inconsistency; you have to infer it
|
|
# from the examples.
|
|
# For this reason, we can't use _apply_rule_list here.
|
|
if word.endswith("e"):
|
|
stem = self._replace_suffix(word, "e", "")
|
|
if self._measure(stem) > 1:
|
|
return stem
|
|
if self._measure(stem) == 1 and not self._ends_cvc(stem):
|
|
return stem
|
|
return word
|
|
|
|
def _step5b(self, word):
|
|
"""Implements Step 5a from "An algorithm for suffix stripping"
|
|
|
|
From the paper:
|
|
|
|
Step 5b
|
|
|
|
(m > 1 and *d and *L) -> single letter
|
|
controll -> control
|
|
roll -> roll
|
|
"""
|
|
return self._apply_rule_list(
|
|
word, [("ll", "l", lambda stem: self._measure(word[:-1]) > 1)]
|
|
)
|
|
|
|
def stem(self, word):
|
|
stem = word.lower()
|
|
|
|
if self.mode == self.NLTK_EXTENSIONS and word in self.pool:
|
|
return self.pool[word]
|
|
|
|
if self.mode != self.ORIGINAL_ALGORITHM and len(word) <= 2:
|
|
# With this line, strings of length 1 or 2 don't go through
|
|
# the stemming process, although no mention is made of this
|
|
# in the published algorithm.
|
|
return word
|
|
|
|
stem = self._step1a(stem)
|
|
stem = self._step1b(stem)
|
|
stem = self._step1c(stem)
|
|
stem = self._step2(stem)
|
|
stem = self._step3(stem)
|
|
stem = self._step4(stem)
|
|
stem = self._step5a(stem)
|
|
stem = self._step5b(stem)
|
|
|
|
return stem
|
|
|
|
def __repr__(self):
|
|
return "<PorterStemmer>"
|
|
|
|
|
|
def demo():
|
|
"""
|
|
A demonstration of the porter stemmer on a sample from
|
|
the Penn Treebank corpus.
|
|
"""
|
|
|
|
from nltk.corpus import treebank
|
|
from nltk import stem
|
|
|
|
stemmer = stem.PorterStemmer()
|
|
|
|
orig = []
|
|
stemmed = []
|
|
for item in treebank.fileids()[:3]:
|
|
for (word, tag) in treebank.tagged_words(item):
|
|
orig.append(word)
|
|
stemmed.append(stemmer.stem(word))
|
|
|
|
# Convert the results to a string, and word-wrap them.
|
|
results = " ".join(stemmed)
|
|
results = re.sub(r"(.{,70})\s", r"\1\n", results + " ").rstrip()
|
|
|
|
# Convert the original to a string, and word wrap it.
|
|
original = " ".join(orig)
|
|
original = re.sub(r"(.{,70})\s", r"\1\n", original + " ").rstrip()
|
|
|
|
# Print the results.
|
|
print("-Original-".center(70).replace(" ", "*").replace("-", " "))
|
|
print(original)
|
|
print("-Results-".center(70).replace(" ", "*").replace("-", " "))
|
|
print(results)
|
|
print("*" * 70)
|