from wordnet import *
from wntools import *

def substituteKeyword(description, keyword):
    words = description.split(' ')
    return string.join( map(lambda word: (keyword, word)[morphy(word, 'noun')==None], words), ' ' )

http://www.blackcodeseo.com/keyword-distribution-over-rss-syndication/

Substituting Nouns With A Keyword With WordNet

Python