another solution to this munging problem

movie_reviews.py

movie_reviews = dict()
for line in open('movies.txt.small'):
  pieces = line.split(':')
  if len(pieces) > 1:
    key = pieces[0]
    value = ':'.join(pieces[1:]).strip()
    if key == 'product/productId':
      id = value
    if key == 'review/text':
      movie_reviews.setdefault(id, []).append(value)
for movie, reviews in movie_reviews.items():
  print movie, reviews

Using 'id' as a variable name is not actually such a good idea, since it's the name of a built-in function, but it works okay here!