input: 
  def largest_smallest_integers(lst):
      '''
      Create a function that returns a tuple (a, b), where 'a' is
      the largest of negative integers, and 'b' is the smallest
      of positive integers in a list.
      If there is no negative or positive integers, return them as None.
  
      Examples:
      largest_smallest_integers([2, 4, 1, 3, 5, 7]) == (None, 1)
      largest_smallest_integers([]) == (None, None)
      largest_smallest_integers([0]) == (None, None)
      '''
  
output:     largest_negative = None
      smallest_positive = None
      
      for num in lst:
          if num < 0:
              if largest_negative is None or num > largest_negative:
                  largest_negative = num
          elif num > 0:
              if smallest_positive is None or num < smallest_positive:
                  smallest_positive = num
      
      return (largest_negative, smallest_positive)