input: 
  def str_to_float(ratings):
      """ Converts a list of string ratings to a list of float ratings.
      Example:
      >>> str_to_float(['4.5', '3.0', '2.5', '4.0'])
      [4.5, 3.0, 2.5, 4.0]
      """
      return [float(x) for x in ratings]
  
output:     return [float(x) for x in ratings]