Continuing with bpo-27589, looks like as_completed documentation is still misleading. According to the docs, it "Return(s) an iterator of Future objects. Each Future object returned represents the earliest result from the set of the remaining awaitables."

There's only one problem: The only thing it definitely doesn't do, is return an iterator of future objects. To be honest with you, I'm not entirely sure how to phrase it.

For reference, I fell for this:

mapping = {fut: index for fut, index in enumerate(futures)}
for fut in as_completed(mapping):
  mapping[fut]  # KeyError