python: how to create an iterator/iterable backed by threaded operation results?

Question

I'm trying to improve a performance issue by preloading data used by a batching iterator, and I'm stuck at the point of fitting this into the python idiomatic style.

Currently I'm using a sequence of iterators composed one on top of the other:

iterator = list(dirin.iterdir())
iterator = TranformIterator(iterator, lambda path: load_img(path)) # This takes most of the time

iterator = PreloadingIterator(iterator) # I want to use this iterator to preload some of the data

iterator = BatchingIterator(iterator, batchsize=100)
iterator = BatchingIteratorWithMeta(iterator)

for batch in iterator:

All of these are implemented with the exception of PreloadingIterator:

class PreloadingIterator:

    inner: typing.Iterator

    def __init__(self,
                 inner: [typing.Union[typing.Sized, typing.Iterable]]
                 ):
        self.inner = inner
        self.total = len(inner)
        self.index = 0
        self.__cache__ = []


    def __len__(self):
        return len(self.inner)

    def __iter__(self):
        mem = psutil.virtual_memory()
        for item in self.inner:
            memconsumed = psutil.virtual_memory()

            if self.should_preload(mem, memconsumed):
                pass
                #threading.Thread(target=
                #peeker = more_itertools.peekable(self.inner)
                #preload_item = peeker.peek()

            yield item

            self.index += 1

    def should_preload(self, oldmem, newmem):
        return True # TODO

What I'm trying to do is peek ahead at the next item in the iterator(preload_item = peeker.peek()) and use that to start a thread to start loading the next result from the iterator. However, I'm struggling to think how I can change the item in for item in self.inner: so it refers to the next item, not the one from the underlying iterator.

How can I iterate over an iterator in a way which allows me to source the item from a precached result if it is available?