R. David Murray added the comment: Yes, the behavior here is "undefined" at the language level. And as pyos says, technically it is documented: if you mutate the object over which you are iterating inside the iteration context (a for loop is just one example of same), the behavior is explicitly undefined. I agree that it is not *obvious* that that applies to chain.from_iterable in this context, but I'm not sure how we'd improve the docs to cover it.
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