words = []
next = raw_input('Start entering words:\n')
while next not in words:
    words.append(next)
    next = raw_input()
print 'You already entered ' + next + '.'
print 'You listed', len(fruits), 'distinct words.'

def sliding(word, num=3):
  for i in range(len(word) + 1 - num):
    print ' ' * i + word[i:i+num]

Here is a version that works if we can assume that the list is non empty, and thus we can legitimately initialize shortSoFar to stringList[0].

def minLength(stringList):
  shortSoFar = stringList[0]                 # use the first entry
  for entry in stringList[1: ]:              # check rest of the list
    if len(entry) < len(shortSoFar):         # for even shorter lengths
      shortSoFar = entry
  return shortSoFar

def minLength(stringList):
  shortSoFar = None
  for entry in stringList:
    if shortSoFar is None or len(entry) < len(shortSoFar):
      shortSoFar = entry
  return shortSoFar

def greaterIndex(data, value):
  answer = 0
  while answer < len(data) and data[answer] <= value:
    answer += 1
  return answer

def convertToBase(value, base):
    result = ''
    while value > 0:
        q = value / base
        r = value % base
        result = str(r) + result
        value = q
    return result