input: 
  from typing import List
  
  def predict_temperature(temperature_list: List[float]) -> float:
      """ Predict the next day's forecast temperature using the exponential smoothing algorithm.
      The prediction is based on the past three day's forecast temperatures.
      >>> predict_temperature([21, 22, 20])
      20.5
      """
      alpha = 0.5
      predicted_temperature = temperature_list[-1]
      for temperature in temperature_list[:-1]:
          predicted_temperature = alpha * temperature + (1 - alpha) * predicted_temperature
      return predicted_temperature
  
output:     alpha = 0.5
      predicted_temperature = temperature_list[-1]
      for temperature in temperature_list[:-1]:
          predicted_temperature = alpha * temperature + (1 - alpha) * predicted_temperature
      return predicted_temperature