input: def extreme_integers(lst):
      '''
      Develop a function that gives back a tuple (x, y), where 'x' denotes
      the maximum negative integer, and 'y' is the minimum positive integer
      found in a list. If no such numbers exist, return None for each.
  
      Examples:
      extreme_integers([2, 4, 1, 3, 5, 7]) == (None, 1)
      extreme_integers([]) == (None, None)
      extreme_integers([0]) == (None, None)
      '''
output:     
      max_negative = None
      min_positive = None
  
      for num in lst:
          if num < 0:
              if max_negative is None or num > max_negative:
                  max_negative = num
          elif num > 0:
              if min_positive is None or num < min_positive:
                  min_positive = num
  
      return (max_negative, min_positive)
  