pylab: TypeError: unhashable type: 'list'

[typetoken]

For the exercise 23 on page 76, or  http://nltk.googlecode.com/svn/trunk/doc/book/ch02.html, 

I try the following codes but it pops up "TypeError: unhashable type: 'list"

>>> import nltk

>>> text = nltk.corpus.brown.words(categories = 'news')

>>> def zif(text):

fdist = nltk.FreqDist(text)

import pylab

word = fdist.keys()

x = []

y = []

index = []

for i in range(0,len(set(text)),1):

x.extend(word[i:i+1])

y.extend(fdist[word[i:i+1]])

index.extend('i+1')

pylab.plot(index, y,'b')

pylab.title('zipf law')

pylab.xlabel('word rank')

pylab.ylabel('word frequency')

pylab.show()

>>> zif(text)

Traceback (most recent call last):

  File "<pyshell#5>", line 1, in <module>

    zif(text)

  File "<pyshell#3>", line 10, in zif

    y.extend(fdist[word[i:i+1]])

  File "C:\Python27\lib\site-packages\nltk\probability.py", line 495, in __getitem__

    return self.get(sample, 0)

TypeError: unhashable type: 'list'

Any suggestions or solutions? thanks indeed.

[John H. Li]

Now the solution has been found: 

For the previous codes, I replace y.extend(fdist[word[i:i+1]]) with y.extend([fdist[word[i]]]). Then it works. I come to realize the difference between y.extend('90') , y.extend(['90']),  y.extend(90) and y.extend([90]).  

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