gistfile1.py

class ReversiableItertor:
    """
    An iterator, having the previous function
    """
    def __init__(self,data):
        """
        """
        self.data = data
        self.index=-1

    def __setPosition(self,position):
        """
        """
        self.index = position

    def getCurrentPosition(self):
        """
        """
        return self.index

    def __iter__(self):
        return self
    
    def previous(self,n=1):
        """
        Returns the previous nth element from the current position,
        None if current position is beyond list lower limit
        """
        prev_pos =  self.index - n
        if prev_pos >= 0:
            self.index = prev_pos
            return self.data[self.index]
        else:
            return None

    def next(self,n=1):
        """
        Returns the next nth element from the current position,
        None if current position is beyond list upper limit
        """
        next_pos =  self.index + n
        if next_pos < len(self.data):
            self.index = next_pos
            return self.data[self.index]
        else:
            return None

    def getPositionToken(self,position):
        """
        """
        if position < len(self.data):
            self.__setPosition(position)
            return self.data[position]
        else:
            return None