input: Write a function to convert a list of string ratings to a list of floats. The input is a list of strings representing numerical ratings, and the output should be a list of floats.
  
  def str_to_float(ratings):
      return [float(x) for x in ratings]
  
  assert str_to_float(['4.5', '3.0', '2.5', '4.0']) == [4.5, 3.0, 2.5, 4.0]
output: def str_to_float(ratings):
      return [float(x) for x in ratings]