input: 
  from typing import List
  
  
  def classify_songs(songs: List['Song']) -> List[str]:
      """ Classify a list of Song objects by their length. The length of a
      song is determined by the length method of the Song class.
      >>> songs = [Song('Bohemian Rhapsody', 'Queen'), Song('Imagine', 'John Lennon')]
      >>> classify_songs(songs)
      ['6', '6']
      """
  
output:     classifications = []
      for song in songs:
          length = song.length()
          if length < 3:
              classifications.append('1')
          elif length < 5:
              classifications.append('2')
          elif length < 7:
              classifications.append('3')
          elif length < 9:
              classifications.append('4')
          elif length < 11:
              classifications.append('5')
          else:
              classifications.append('6')
      return classifications