input: Write a function to convert ratings in a list of strings to a float. The function should take a list of strings as input and return 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]