The chapter shows how a dictionary provides a clever way to count the number of unique words. For those familiar with sets, there's yet an easier way to do this (using Python's set type).

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def uCount(fname):
  F = open(fname)
  ewords = [ w in F.read().split()
              if 'e' in w ]
  uniqs = set(words)
  p = 100*len(eWords)/float(len(uniqs))
  return int(p)

Line 3 (actually lines 3 and 4) makes a list of all the "e"-words in a file, and line 5 defines the set of such words.