I don't think you need WordNet to find proper nouns, I suggest using the Part-Of-Speech tagger pos_tag.
To find Proper Nouns, look for the NNP tag:
from nltk.tag import pos_tag
sentence = "Michael Jackson likes to eat at McDonalds"
tagged_sent = pos_tag(sentence.split())
# [('Michael', 'NNP'), ('Jackson', 'NNP'), ('likes', 'VBZ'), ('to', 'TO'), ('eat', 'VB'), ('at', 'IN'), ('McDonalds', 'NNP')]
propernouns = [word for word,pos in tagged_sent if pos == 'NNP']
# ['Michael','Jackson', 'McDonalds']
You may not be very satisfied since Michael and Jackson is split into 2 tokens, then you might need something more complex such as Name Entity tagger.
By right, as documented by the penntreebank tagset, for possessive nouns, you can simply look for the POS tag, http://www.mozart-oz.org/mogul/doc/lager/brill-tagger/penn.html. But often the tagger doesn't tag POS when it's an NNP.
To find Possessive Nouns, look for str.endswith("'s") or str.endswith("s'"):
from nltk.tag import pos_tag
sentence = "Michael Jackson took Daniel Jackson's hamburger and Agnes' fries"
tagged_sent = pos_tag(sentence.split())
# [('Michael', 'NNP'), ('Jackson', 'NNP'), ('took', 'VBD'), ('Daniel', 'NNP'), ("Jackson's", 'NNP'), ('hamburger', 'NN'), ('and', 'CC'), ("Agnes'", 'NNP'), ('fries', 'NNS')]
possessives = [word for word in sentence if word.endswith("'s") or word.endswith("s'")]
# ["Jackson's", "Agnes'"]
Alternatively, you can use NLTK ne_chunk but it doesn't seem to do much other unless you are concerned about what kind of Proper Noun you get from the sentence:
>>> from nltk.tree import Tree; from nltk.chunk import ne_chunk
>>> [chunk for chunk in ne_chunk(tagged_sent) if isinstance(chunk, Tree)]
[Tree('PERSON', [('Michael', 'NNP')]), Tree('PERSON', [('Jackson', 'NNP')]), Tree('PERSON', [('Daniel', 'NNP')])]
>>> [i[0] for i in list(chain(*[chunk.leaves() for chunk in ne_chunk(tagged_sent) if isinstance(chunk, Tree)]))]
['Michael', 'Jackson', 'Daniel']
Using ne_chunk is a little verbose and it doesn't get you the possessives.
