input: Write a python function to retrieve the maximum positive number in the provided list.
  assert largest_pos([1,2,3,4,-1]) == 4
  assert largest_pos([0,1,2,-5,-1,6]) == 6
  assert largest_pos([0,0,1,0]) == 1
output: def largest_pos(lst):
      return max([num for num in lst if num > 0], default=None)