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This would be hard to get right, but maybe not impossible. If the frame is suspended, and will do special handling of GeneratorExit, a ResourceWarning would be nice in most cases. But if the frame will not catch any exception, there should be no ResourceWarning. I think this is the big difference between mygen() and Victor’s generator expression:
gen = (2**i for i in itertools.count())
next(gen) # Generator suspended but won’t catch any exception
del gen # Like deleting a plain iterator; no ResourceWarning expected
One more complication: If the frame is suspended at something like "yield from open(fname)", it should trigger a ResourceWarning, because GeneratorExit would cause the file’s close() method to be called. But if "yield from" is calling a simpler generator-iterator with nothing to clean up, it should not emit a ResourceWarning.