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