'yield from' rewrite unsafe when looping variable is referenced later

Consider the example code:

def foo(it):
    item = None
    for item in it:
        yield item
    print("Last item", item)

With --py3-plus, this is transformed to:

def foo(it):
    item = None
    yield from it
    print("Last item", item)

But these are not equivalent. The line print("Last item", item) now always prints None rather than the last item of the passed iterable.

This was discovered in more-itertools:

https://github.com/erikrose/more-itertools/blob/8d860c355a1dd900e3a9c5fe1303ddf5a778452f/more_itertools/more.py#L1206-L1223

[asottile]

I’ve picked up work again on this in #286 – it’s getting there but needs more scope handling

[asottile]

it’s going to be a little tricky, but not impossible – basically the ast visitor needs to check if the loop variable(s) are referenced after the loop – you might find some prior art from flake8-bugbear which has a lint check for this