1  
       2  _NOT_SET = object()
       3  
       4  
       5  def peek_and_iter(items):
       6      if not items:
       7          return None, None
       8      items = iter(items)
       9      try:
      10          peeked = next(items)
      11      except StopIteration:
      12          return None, None
      13      def chain():
      14          yield peeked
      15          yield from items
      16      return chain(), peeked
      17  
      18  
      19  def iter_many(items, onempty=None):
      20      if not items:
      21          if onempty is None:
      22              return
      23          if not callable(onempty):
      24              raise onEmpty
      25          items = onempty(items)
      26          yield from iter_many(items, onempty=None)
      27          return
      28      items = iter(items)
      29      try:
      30          first = next(items)
      31      except StopIteration:
      32          if onempty is None:
      33              return
      34          if not callable(onempty):
      35              raise onEmpty
      36          items = onempty(items)
      37          yield from iter_many(items, onempty=None)
      38      else:
      39          try:
      40              second = next(items)
      41          except StopIteration:
      42              yield first, False
      43              return
      44          else:
      45              yield first, True
      46              yield second, True
      47          for item in items:
      48              yield item, True