input: Write a function to find and print the maximum number from a given list. If the list is empty, return None.
  
  assert max_num([5, 4, 2, 7, 12]) == 12
  assert max_num([10, 3, 7]) == 10
  assert max_num([1]) == 1
  assert max_num([]) == None
output: def max_num(lst):
      if not lst:
          return None
      return max(lst)