Non-None return value from generator should throw RuntimeError

[jvm-pangea]

Bug report

Bug description:

def y1(*args):
    return args

def y2(*args):
    return args
    yield 3

print("y1: " + str(list(y1(1, 2, 3, 4))))
print("y2: " + str(list(y2(1, 2, 3, 4))))

While the yield in y2 is unreachable (this happened to me while re-writing a function and leaving what I thought was dead code at the end), y2 is considered a Generator, and iterating over it produces an immediate StopIteration error with args as the argument, which manifests as an empty iterator.

I can't imagine a good reason to create a StopIteration exception with a non-None argument. With a None argument, this signals the basic case where a Generator is returning an empty iterator.

Consider throwing RuntimeError when StopIteration is raised with a non-None valued error when calling next to prevent this confusing behaviour.

CPython versions tested on:

3.10

Operating systems tested on:

macOS

[brianschubert]

Hi jvm-pangea, using return in generators is an intentional feature. You can see PEP 380 for some of the original motivation and use cases.

[nineteendo]

This feels like a problem for a linter to flag.

[jvm-pangea]

PEP 380 will take some time for me to grok, it's pretty dense.

Is there no way to know when executing the byte-code that you're handling a StopIteration in the context of a yield from (where it is expected) vs. in a naked generator where the non-None valued exception would clearly be problematic?

[picnixz]

I agree that this is more a linter issue rather than a core issue. If you want to change the current behaviour a PEP is likely needed or at least a d.p.o thread (https://discuss.python.org/c/ideas/6).

[jvm-pangea]

Opened discussion:

https://discuss.python.org/t/foot-gun-no-runtimeerror-on-non-none-return-from-generator-outside-of-yield-from/72579