pushback_iter.py

import collections

class pushback_iter:
    """
    Wraps an iterable to allow pushing back previous values
    """
    def __init__(self, it):
        self.it = it
        self.buffer = collections.deque()

    def __next__(self):
        if len(self.buffer):
            return self.buffer.pop()
        return next(self.it)

    def __iter__(self):
        return self

    def push(self, line):
        self.buffer.append(line)