This is my code

import nltk
freq_dist = nltk.FreqDist(words)
print freq_dist.keys()[:50] # 50 most frequent tokens
print freq_dist.keys()[-50:] # 50 least frequent tokens

And I am getting this error message:

ImportError: cannot import name compat

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You're trying to import the entirety of nltk when you probably only need a small portion. try editing your code to

from nltk import FreqDist
freq_dist = FreqDist(words)
print freq_dist.keys()[:50] # 50 most frequent tokens
print freq_dist.keys()[-50:] # 50 least frequent tokens