input: Write a function to calculate the average of a list of positive integers and output the result. If the list is empty, return 0.
  assert average_of_positives([1, 5, 6, 7, 8]) == 5.4
  assert average_of_positives([3, 9, 2]) == 4.666666666666667
  assert average_of_positives([0]) == 0
  assert average_of_positives([]) == 0
output: def average_of_positives(lst):
      if not lst:
          return 0
      positives = [x for x in lst if x > 0]
      if not positives:
          return 0
      return sum(positives) / len(positives)