On Sat, Nov 25, 2017 at 6:55 AM, Ivan Levkivskyi <levkivskyi@gmail.com> wrote:

On 25 November 2017 at 04:30, Guido van Rossum <guido@python.org> wrote:

On Fri, Nov 24, 2017 at 4:22 PM, Guido van Rossum <guido@python.org> wrote:

The more I hear about this topic, the more I think that `await`, `yield` and `yield from` should all be banned from occurring in all comprehensions and generator expressions. That's not much different from disallowing `return` or `break`.

From the responses it seems that I tried to simplify things too far. Let's say that `await` in comprehensions is fine, as long as that comprehension is contained in an `async def`. While we *could* save `yield [from]` in comprehensions, I still see it as mostly a source of confusion, and the fact that the presence of `yield [from]` *implicitly* makes the surrounding `def` a generator makes things worse. It just requires too many mental contortions to figure out what it does.

[...]

If the first example will be allowed, then one will be surprised why it can't be rewritten as

    def pack_two():

        return [(yield) for _ in range(2)]

And yet Nick's example shows that that is not equivalent!

[...]

In this example each thing that looks syntactically like a list comprehension becomes actually a generator expression at at runtime! And so does your example, so instead of a list of two items, it returns a generator that will produce two values when iterated over.

That's not referential transparency to me, it feels more like a bug in the code generator.

I want to ban this because apparently nobody besides Nick knows about this behavior (I certainly didn't, and from the above it seems you don't either).