On 25 November 2017 at 16:57, Guido van Rossum <guido@python.org> wrote:
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).
This whole thread started as a proposal to fix this bug and to make the two forms equivalent, so I don't know what you are talking about. Also as there appeared arguments of authority (thanks Antoine) its time to stop this discussion for me. -- Ivan