[PorterStemmer] Remove _adjust_case for consistency with Lancaster and Snowball

ExplodingCabbage · ExplodingCabbage · commit a2d1dfa7351f · 2016-01-10T18:41:22.000Z
Handling of upper and lower case is not specified in Martin Porter's "An algorithm for suffix stripping" paper; the algorithm description there never even mentions the existence of difference letter cases. Nor does Martin's C implementation of the stemmer at: http://tartarus.org/~martin/PorterStemmer/c.txt handle case in the way that NLTK's version has been doing; instead, it simply requires that the user convert their word to lowercase before calling stem(). Since there is no Porter-specific reason to preserve our (odd) behaviour here, and our other StemmerI implementations don't do it, we should probably purge it, as this commit does.
diff --git a/nltk/stem/porter.py b/nltk/stem/porter.py
@@ -529,18 +529,6 @@ def _step5(self, word):
 
         return word
 
-    def _adjust_case(self, word, stem):
-        lower = word.lower()
-
-        ret = ""
-        for x in range(len(stem)):
-            if lower[x] == stem[x]:
-                ret += word[x]
-            else:
-                ret += stem[x]
-
-        return ret
-
     def stem(self, word):
         stem = word.lower()
         
@@ -562,7 +550,7 @@ def stem(self, word):
         stem = self._step4(stem)
         stem = self._step5(stem)
         
-        return self._adjust_case(word, stem)
+        return stem
 
     def __repr__(self):
         return '<PorterStemmer>'
