I think what you are looking for is the stopwords.words from nltk.corpus:
>>> from nltk.corpus import stopwords
>>> sw = set(stopwords.words('english'))
>>> sentence = "a long sentence that contains a for instance"
>>> [w for w in sentence.split() if w not in sw]
['long', 'sentence', 'contains', 'instance']
Edit: searching for stopword give possible duplicates: Stopword removal with NLTK, How to remove stop words using nltk or python. See the answers of these question. And consider Effects of Stemming on the term frequency? too