WordNetLemmatizer 使用._morphy 函数来访问其单词的引理；来自http://www.nltk.org/_modules/nltk/stem/wordnet.html 并返回最小长度的可能引理。
def lemmatize(self, word, pos=NOUN):
lemmas = wordnet._morphy(word, pos)
return min(lemmas, key=len) if lemmas else word
._morphy 函数迭代地应用规则以获得引理；规则不断减少单词的长度并将词缀替换为MORPHOLOGICAL_SUBSTITUTIONS。然后看是否还有其他更短但与缩减词相同的词：
def _morphy(self, form, pos):
# from jordanbg:
# Given an original string x
# 1. Apply rules once to the input to get y1, y2, y3, etc.
# 2. Return all that are in the database
# 3. If there are no matches, keep applying rules until you either
# find a match or you can't go any further
exceptions = self._exception_map[pos]
substitutions = self.MORPHOLOGICAL_SUBSTITUTIONS[pos]
def apply_rules(forms):
return [form[:-len(old)] + new
for form in forms
for old, new in substitutions
if form.endswith(old)]
def filter_forms(forms):
result = []
seen = set()
for form in forms:
if form in self._lemma_pos_offset_map:
if pos in self._lemma_pos_offset_map[form]:
if form not in seen:
result.append(form)
seen.add(form)
return result
# 0. Check the exception lists
if form in exceptions:
return filter_forms([form] + exceptions[form])
# 1. Apply rules once to the input to get y1, y2, y3, etc.
forms = apply_rules([form])
# 2. Return all that are in the database (and check the original too)
results = filter_forms([form] + forms)
if results:
return results
# 3. If there are no matches, keep applying rules until we find a match
while forms:
forms = apply_rules(forms)
results = filter_forms(forms)
if results:
return results
# Return an empty list if we can't find anything
return []
但是如果单词在例外列表中，它会返回一个保存在exceptions 中的固定值，参见http://www.nltk.org/_modules/nltk/corpus/reader/wordnet.html 中的_load_exception_map：
def _load_exception_map(self):
# load the exception file data into memory
for pos, suffix in self._FILEMAP.items():
self._exception_map[pos] = {}
for line in self.open('%s.exc' % suffix):
terms = line.split()
self._exception_map[pos][terms[0]] = terms[1:]
self._exception_map[ADJ_SAT] = self._exception_map[ADJ]
回到你的例子，worse -> bad 和 further -> far 不能从规则中实现，因此它必须来自例外列表。由于它是一个例外列表，因此必然存在不一致之处。
例外列表保存在~/nltk_data/corpora/wordnet/adv.exc 和~/nltk_data/corpora/wordnet/adv.exc。
来自adv.exc：
best well
better well
deeper deeply
farther far
further far
harder hard
hardest hard
来自adj.exc：
...
worldliest worldly
wormier wormy
wormiest wormy
worse bad
worst bad
worthier worthy
worthiest worthy
wrier wry
...