[issue10544] yield expression inside generator expression does nothing

[Nick Coghlan]

Nick Coghlan <ncoghlan at gmail.com> added the comment:

I realised that even without modifying the compiler first, I could illustrate the proposed `yield from` based resolution for the comprehension case by way of explicit yield from clauses:

```
def get_gen_result(gen, inputs):
    try:
        yield_value = next(gen)
        for send_value in inputs:
            print(f"Received: {yield_value}; Sending: {send_value}")
            yield_value = gen.send(send_value)
    except StopIteration as exc:
        return exc.value
    raise RuntimeError("Failed to exhaust generator")

def example():
    comp1 = yield from [str((yield x)) for x in ('1st', '2nd')]
    comp2 = yield from [int((yield x)) for x in ('3rd', '4th')]
    return comp1, comp2

>>> result = get_gen_result(example(), range(4))
Received: 1st; Sending: 0
Received: 2nd; Sending: 1
Received: 3rd; Sending: 2
Received: 4th; Sending: 3
>>> result
(['0', '1'], [2, 3])
```

So if we decided to make yield-in-a-comprehension imply the use of yield from, we'd only need:

- DeprecationWarning in 3.7 to say "this is going to imply 'yield from (comprehension)' in 3.8+"
- making the 'yield from' implicit in 3.8 (thus ensuring that comprehensions always return the correct container type, even when they include yield expressions)

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue10544>
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