last_loop_cacher.py

class LastLoopCacherNoFinal:
    pass


class LastLoopCacher:
    """
    An iterable context manager that stores the last value it loops as
    an attribute, `final`.
    Final falls out of scope upon exiting the context manager.

    For use in cases where many objects are looped but not all of their attrs
    are of iterest. Normally this requires you to contiuously repeatedly store
    the same data over and over again, but with this we can catch and cache the
    last one.
    """
    def __init__(self, xs):
        self.xs = xs
        self._final = LastLoopCacherNoFinal

    def __iter__(self):
        ix = iter(self.xs)
        while True:
            try:
                x = next(ix)
                yield x
            except StopIteration:
                try:
                    self._final = x
                    raise StopIteration
                except UnboundLocalError:
                    raise RuntimeError("Cannot rely on cache of empty iterable.")

    def __enter__(self):
        return self

    def __exit__(self, type_, value, traceback):
        self.reset()

    @property
    def final(self):
        if self._final is LastLoopCacherNoFinal:
            raise RuntimeError("Cannot access final loop out of scope.")
        return self._final

    def reset(self):
        self._final = LastLoopCacherNoFinal