input: Write a function to calculate the maximum, minimum, and average values from a given sequence of numbers.
  
  def calculate_metrics(list):
      maximum = max(list)
      minimum = min(list)
      n = len(list)
      total = 0
      for num in list:
          total += num
      average = total/n
      return maximum, minimum, average
  
  assert calculate_metrics([3, 4, 5, 6, 21, 8]) == (21, 3, 8.166666666666666)
  assert calculate_metrics([1, 1, 1, 1]) == (1, 1, 1.0)
  assert calculate_metrics([-10, 0, 10]) == (10, -10, 0.0)
  assert calculate_metrics([]) == (None, None, None)
  assert calculate_metrics([7]) == (7, 7, 7.0)
output: def calculate_metrics(list):
      if not list:
          return None, None, None
      maximum = max(list)
      minimum = min(list)
      average = sum(list) / len(list)
      return maximum, minimum, average