Python Porter Stemmer Reference
This is a common porter stemmer implementation in Python that I've modified here and there over the years.
import re
from functools import reduce
STOP_WORDS = r"""\b(a|about|above|across|after|afterwards|again|against|all|almost|alone|along|already|also|
although|always|am|among|amongst|amoungst|amount|an|and|another|any|anyhow|anyone|anything|anyway|anywhere|are|
around|as|at|back|be|became|because|become|becomes|becoming|been|before|beforehand|behind|being|below|beside|
besides|between|beyond|bill|both|bottom|but|by|call|can|cannot|cant|co|computer|con|could|couldnt|de|describe|
detail|do|done|down|due|during|each|eg|eight|either|eleven|else|elsewhere|empty|enough|etc|even|ever|every|everyone|
everything|everywhere|except|few|fifteen|fifty|fill|find|fire|first|five|for|former|formerly|forty|found|four|from|
front|full|further|get|give|go|had|has|hasnt|have|he|hence|her|here|hereafter|hereby|herein|hereupon|hers|herself|
him|himself|his|how|however|hundred|i|ie|if|in|inc|indeed|interest|into|is|it|its|itself|last|latter|latterly|
least|less|ltd|made|many|may|me|meanwhile|might|mill|mine|more|moreover|most|mostly|move|much|must|my|myself|name|
namely|neither|never|nevertheless|next|nine|no|nobody|none|noone|nor|not|nothing|now|nowhere|of|off|often|on|once|
one|only|onto|or|other|others|otherwise|our|ours|ourselves|out|over|own|part|per|perhaps|please|put|rather|re|same|
see|seem|seemed|seeming|seems|serious|several|she|should|show|side|since|sincere|six|sixty|so|some|somehow|someone|
something|sometime|sometimes|somewhere|still|such|system|take|ten|than|that|the|their|them|themselves|then|thence|
there|thereafter|thereby|therefore|therein|thereupon|these|they|third|this|those|though|three|through|
throughout|thru|thus|to|together|too|top|toward|towards|twelve|twenty|two|un|under|until|up|upon|us|very|via|was|
we|well|were|what|whatever|when|whence|whenever|where|whereafter|whereas|whereby|wherein|whereupon|wherever|whether|
which|while|whither|who|whoever|whole|whom|whose|why|will|with|within|without|would|yet|you|your|yours|yourself|
yourselves)\b"""
stop_word_list = re.compile(STOP_WORDS.replace('\n', ''), re.IGNORECASE)
re_filter = re.compile('[\W_]+')
def stem_phrase(text):
stemmer = PorterStemmer()
stemmed = ''
text = text.replace('-', ' ')
text = re.sub('</?[^>]+>', '', text) # Remove HTML
text = stop_word_list.sub('', text)
words = text.split(' ')
for word in words:
word = re_filter.sub('', word.lower())
if len(word) < 2:
continue
else:
word = stemmer.stem(word, 0, len(word)-1)
stemmed += ' %s' % word
return stemmed.strip()
class PorterStemmer:
def __init__(self):
"""The main part of the stemming algorithm starts here.
b is a buffer holding a word to be stemmed. The letters are in b[k0],
b[k0+1] ... ending at b[k]. In fact k0 = 0 in this demo program. k is
readjusted downwards as the stemming progresses. Zero termination is
not in fact used in the algorithm.
Note that only lower case sequences are stemmed. Forcing to lower case
should be done before stem(...) is called.
"""
self.b = "" # buffer for word to be stemmed
self.k = 0
self.k0 = 0
self.j = 0 # j is a general offset into the string
def cons(self, i):
"""cons(i) is TRUE <=> b[i] is a consonant."""
if self.b[i] == 'a' or self.b[i] == 'e' or self.b[i] == 'i' or self.b[i] == 'o' or self.b[i] == 'u':
return 0
if self.b[i] == 'y':
if i == self.k0:
return 1
else:
return (not self.cons(i - 1))
return 1
def m(self):
"""m() measures the number of consonant sequences between k0 and j.
if c is a consonant sequence and v a vowel sequence, and <..>
indicates arbitrary presence,
<c><v> gives 0
<c>vc<v> gives 1
<c>vcvc<v> gives 2
<c>vcvcvc<v> gives 3
....
"""
n = 0
i = self.k0
while 1:
if i > self.j:
return n
if not self.cons(i):
break
i = i + 1
i = i + 1
while 1:
while 1:
if i > self.j:
return n
if self.cons(i):
break
i = i + 1
i = i + 1
n = n + 1
while 1:
if i > self.j:
return n
if not self.cons(i):
break
i = i + 1
i = i + 1
def vowelinstem(self):
"""vowelinstem() is TRUE <=> k0,...j contains a vowel"""
for i in range(self.k0, self.j + 1):
if not self.cons(i):
return 1
return 0
def doublec(self, j):
"""doublec(j) is TRUE <=> j,(j-1) contain a double consonant."""
if j < (self.k0 + 1):
return 0
if (self.b[j] != self.b[j-1]):
return 0
return self.cons(j)
def cvc(self, i):
"""cvc(i) is TRUE <=> i-2,i-1,i has the form consonant - vowel - consonant
and also if the second c is not w,x or y. this is used when trying to
restore an e at the end of a short e.g.
cav(e), lov(e), hop(e), crim(e), but
snow, box, tray.
"""
if i < (self.k0 + 2) or not self.cons(i) or self.cons(i-1) or not self.cons(i-2):
return 0
ch = self.b[i]
if ch == 'w' or ch == 'x' or ch == 'y':
return 0
return 1
def ends(self, s):
"""ends(s) is TRUE <=> k0,...k ends with the string s."""
length = len(s)
if s[length - 1] != self.b[self.k]: # tiny speed-up
return 0
if length > (self.k - self.k0 + 1):
return 0
if self.b[self.k-length+1:self.k+1] != s:
return 0
self.j = self.k - length
return 1
def setto(self, s):
"""setto(s) sets (j+1),...k to the characters in the string s, readjusting k."""
length = len(s)
self.b = self.b[:self.j+1] + s + self.b[self.j+length+1:]
self.k = self.j + length
def r(self, s):
"""r(s) is used further down."""
if self.m() > 0:
self.setto(s)
def step1ab(self):
"""step1ab() gets rid of plurals and -ed or -ing. e.g.
caresses -> caress
ponies -> poni
ties -> ti
caress -> caress
cats -> cat
feed -> feed
agreed -> agree
disabled -> disable
matting -> mat
mating -> mate
meeting -> meet
milling -> mill
messing -> mess
meetings -> meet
"""
if self.b[self.k] == 's':
if self.ends("sses"):
self.k = self.k - 2
elif self.ends("ies"):
self.setto("i")
elif self.b[self.k - 1] != 's':
self.k = self.k - 1
if self.ends("eed"):
if self.m() > 0:
self.k = self.k - 1
elif (self.ends("ed") or self.ends("ing")) and self.vowelinstem():
self.k = self.j
if self.ends("at"): self.setto("ate")
elif self.ends("bl"): self.setto("ble")
elif self.ends("iz"): self.setto("ize")
elif self.doublec(self.k):
self.k = self.k - 1
ch = self.b[self.k]
if ch == 'l' or ch == 's' or ch == 'z':
self.k = self.k + 1
elif (self.m() == 1 and self.cvc(self.k)):
self.setto("e")
def step1c(self):
"""step1c() turns terminal y to i when there is another vowel in the stem."""
if (self.ends("y") and self.vowelinstem()):
self.b = self.b[:self.k] + 'i' + self.b[self.k+1:]
def step2(self):
"""step2() maps double suffices to single ones.
so -ization ( = -ize plus -ation) maps to -ize etc. note that the
string before the suffix must give m() > 0.
"""
if self.b[self.k - 1] == 'a':
if self.ends("ational"): self.r("ate")
elif self.ends("tional"): self.r("tion")
elif self.b[self.k - 1] == 'c':
if self.ends("enci"): self.r("ence")
elif self.ends("anci"): self.r("ance")
elif self.b[self.k - 1] == 'e':
if self.ends("izer"): self.r("ize")
elif self.b[self.k - 1] == 'l':
if self.ends("bli"): self.r("ble") # --DEPARTURE--
# To match the published algorithm, replace this phrase with
# if self.ends("abli"): self.r("able")
elif self.ends("alli"): self.r("al")
elif self.ends("entli"): self.r("ent")
elif self.ends("eli"): self.r("e")
elif self.ends("ousli"): self.r("ous")
elif self.b[self.k - 1] == 'o':
if self.ends("ization"): self.r("ize")
elif self.ends("ation"): self.r("ate")
elif self.ends("ator"): self.r("ate")
elif self.b[self.k - 1] == 's':
if self.ends("alism"): self.r("al")
elif self.ends("iveness"): self.r("ive")
elif self.ends("fulness"): self.r("ful")
elif self.ends("ousness"): self.r("ous")
elif self.b[self.k - 1] == 't':
if self.ends("aliti"): self.r("al")
elif self.ends("iviti"): self.r("ive")
elif self.ends("biliti"): self.r("ble")
elif self.b[self.k - 1] == 'g': # --DEPARTURE--
if self.ends("logi"): self.r("log")
# To match the published algorithm, delete this phrase
def step3(self):
"""step3() dels with -ic-, -full, -ness etc. similar strategy to step2."""
if self.b[self.k] == 'e':
if self.ends("icate"): self.r("ic")
elif self.ends("ative"): self.r("")
elif self.ends("alize"): self.r("al")
elif self.b[self.k] == 'i':
if self.ends("iciti"): self.r("ic")
elif self.b[self.k] == 'l':
if self.ends("ical"): self.r("ic")
elif self.ends("ful"): self.r("")
elif self.b[self.k] == 's':
if self.ends("ness"): self.r("")
def step4(self):
"""step4() takes off -ant, -ence etc., in context <c>vcvc<v>."""
if self.b[self.k - 1] == 'a':
if self.ends("al"): pass
else: return
elif self.b[self.k - 1] == 'c':
if self.ends("ance"): pass
elif self.ends("ence"): pass
else: return
elif self.b[self.k - 1] == 'e':
if self.ends("er"): pass
else: return
elif self.b[self.k - 1] == 'i':
if self.ends("ic"): pass
else: return
elif self.b[self.k - 1] == 'l':
if self.ends("able"): pass
elif self.ends("ible"): pass
else: return
elif self.b[self.k - 1] == 'n':
if self.ends("ant"): pass
elif self.ends("ement"): pass
elif self.ends("ment"): pass
elif self.ends("ent"): pass
else: return
elif self.b[self.k - 1] == 'o':
if self.ends("ion") and (self.b[self.j] == 's' or self.b[self.j] == 't'): pass
elif self.ends("ou"): pass
# takes care of -ous
else: return
elif self.b[self.k - 1] == 's':
if self.ends("ism"): pass
else: return
elif self.b[self.k - 1] == 't':
if self.ends("ate"): pass
elif self.ends("iti"): pass
else: return
elif self.b[self.k - 1] == 'u':
if self.ends("ous"): pass
else: return
elif self.b[self.k - 1] == 'v':
if self.ends("ive"): pass
else: return
elif self.b[self.k - 1] == 'z':
if self.ends("ize"): pass
else: return
else:
return
if self.m() > 1:
self.k = self.j
def step5(self):
"""step5() removes a final -e if m() > 1, and changes -ll to -l if
m() > 1.
"""
self.j = self.k
if self.b[self.k] == 'e':
a = self.m()
if a > 1 or (a == 1 and not self.cvc(self.k-1)):
self.k = self.k - 1
if self.b[self.k] == 'l' and self.doublec(self.k) and self.m() > 1:
self.k = self.k -1
def stem(self, p, i, j):
"""In stem(p,i,j), p is a char pointer, and the string to be stemmed
is from p[i] to p[j] inclusive. Typically i is zero and j is the
offset to the last character of a string, (p[j+1] == '\0'). The
stemmer adjusts the characters p[i] ... p[j] and returns the new
end-point of the string, k. Stemming never increases word length, so
i <= k <= j. To turn the stemmer into a module, declare 'stem' as
extern, and delete the remainder of this file.
"""
# copy the parameters into statics
self.b = p
self.k = j
self.k0 = i
if self.k <= self.k0 + 1:
return self.b # --DEPARTURE--
# 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. Remove the line to match the published
# algorithm.
self.step1ab()
self.step1c()
self.step2()
self.step3()
self.step4()
self.step5()
return self.b[self.k0:self.k+1]