async enumeration - possible?

[Ian Kelly]

On Tue, Nov 29, 2016 at 8:22 PM, Chris Angelico <rosuav at gmail.com> wrote:
> Interestingly, I can't do that in a list comp:
>
>>>> [x async for x in aiterable]
>   File "<stdin>", line 1
>     [x async for x in aiterable]
>            ^
> SyntaxError: invalid syntax
>
> Not sure why.

Because you tried to use an async comprehension outside of a coroutine.

py> [x async for x in y]
  File "<stdin>", line 1
    [x async for x in y]
           ^
SyntaxError: invalid syntax
py> async def foo():
...     [x async for x in y]
...

The same is true for async generator expressions. The documentation is
clear that this is illegal for the async for statement:

https://docs.python.org/3.6/reference/compound_stmts.html#the-async-for-statement

I don't see anything about async comprehensions or async generators
outside of the "what's new" section, but it stands to reason that the
same would apply.

On Tue, Nov 29, 2016 at 11:06 PM, Frank Millman <frank at chagford.com> wrote:
>>>> async def main():
>
> ...   print(list(x async for x in gen(5)))
>
> ...
>>>>
>>>> loop.run_until_complete(main())
>
> Traceback (most recent call last):
>  File "<stdin>", line 1, in <module>
>  File
> "C:\Users\User\AppData\Local\Programs\Python\Python36\lib\asyncio\base_events.py",
> line 466, in run_until_complete
>    return future.result()
> TypeError: 'async_generator' object is not iterable

Yeah, that's what I would expect. (x async for x in foo) is
essentially a no-op, just like its synchronous equivalent; it takes an
asynchronous iterator and produces an equivalent asynchronous
iterator. Meanwhile, list() can't consume an async iterator because
the list constructor isn't a coroutine. I don't think it's generally
possible to "synchronify" an async iterator other than to materialize
it. E.g.:

def alist(aiterable):
    result = []
    async for value in aiterable:
        result.append(value)
    return result

And I find it a little disturbing that I actually can't see a better
way to build a list from an async iterator than that.