NLTK WordNet Lemmatizer: Shouldn't it lemmatize all inflections of a word?

The WordNet lemmatizer does take the POS tag into account, but it doesn't magically determine it:

>>> nltk.stem.WordNetLemmatizer().lemmatize('loving')
'loving'
>>> nltk.stem.WordNetLemmatizer().lemmatize('loving', 'v')
u'love'

Without a POS tag, it assumes everything you feed it is a noun. So here it thinks you're passing it the noun "loving" (as in "sweet loving").

The best way to troubleshoot this is to actually look in Wordnet. Take a look here: Loving in wordnet. As you can see, there is actually an adjective "loving" present in Wordnet. As a matter of fact, there is even the adverb "lovingly": lovingly in Wordnet. Because wordnet doesn't actually know what part of speech you actually want, it defaults to noun ('n' in Wordnet). If you are using Penn Treebank tag set, here's some handy function for transforming Penn to WN tags:

from nltk.corpus import wordnet as wn

def is_noun(tag):
    return tag in ['NN', 'NNS', 'NNP', 'NNPS']


def is_verb(tag):
    return tag in ['VB', 'VBD', 'VBG', 'VBN', 'VBP', 'VBZ']


def is_adverb(tag):
    return tag in ['RB', 'RBR', 'RBS']


def is_adjective(tag):
    return tag in ['JJ', 'JJR', 'JJS']


def penn_to_wn(tag):
    if is_adjective(tag):
        return wn.ADJ
    elif is_noun(tag):
        return wn.NOUN
    elif is_adverb(tag):
        return wn.ADV
    elif is_verb(tag):
        return wn.VERB
    return None

Hope this helps.

it's clearer and more effective than enumeration：

from nltk.corpus import wordnet

def get_wordnet_pos(self, treebank_tag):
    if treebank_tag.startswith('J'):
        return wordnet.ADJ
    elif treebank_tag.startswith('V'):
        return wordnet.VERB
    elif treebank_tag.startswith('N'):
        return wordnet.NOUN
    elif treebank_tag.startswith('R'):
        return wordnet.ADV
    else:
        return ''

def penn_to_wn(tag):
    return get_wordnet_pos(tag)