def _fill ( self ) : try : self . _head = self . _iterable . next ( ) except StopIteration : self . _head = None
python change the iterator
Advance the iterator without returning the old head .
def _fill(self):
    """Advance the iterator without returning the old head."""
    try:
      self._head = self._iterable.next()
    except StopIteration:
      self._head = None