code: def random_sample(value, field, row, num=10):
      
      # TODO: Could become a more general DQ piece.
      if value is None:
          field['has_nulls'] = True
          return
      if value in field['samples']:
          return
      if isinstance(value, basestring) and not len(value.strip()):
          field['has_empty'] = True
          return
      if len(field['samples']) < num:
          field['samples'].append(value)
          return
      j = random.randint(0, row)
      if j < (num - 1):
          field['samples'][j] = value
docs: Collect a random sample of the values in a particular
      field based on the reservoir sampling technique.