[Python-Dev] Tricky way of of creating a generator via a comprehension expression

[Nick Coghlan]

On 27 November 2017 at 06:29, Nathaniel Smith <njs at pobox.com> wrote:
> - In async/await, it's not obvious how to write leaf functions:
> 'await' is equivalent to 'yield from', but there's no equivalent to
> 'yield'. You have to jump through some hoops by writing a class with a
> custom __await__ method or using @types.coroutine. Of course it's
> doable, and it's no big deal if you're writing a proper async library,
> but it's awkward for quick ad hoc usage.
>
> For a concrete example of 'ad hoc coroutines' where I think 'yield
> from' is appropriate, here's wsproto's old 'yield from'-based
> incremental websocket protocol parser:
>
>     https://github.com/python-hyper/wsproto/blob/4b7db502cc0568ab2354798552148dadd563a4e3/wsproto/frame_protocol.py#L142

sys.set_coroutine_wrapper itself is another case where you genuinely
*can't* rely on async/await in the wrapper implementation.

The current example shown at
https://docs.python.org/3/library/sys.html#sys.set_coroutine_wrapper
is of a case that will *fail*, since it would otherwise result in
infinite recursion when the coroutine wrapper attempts to call the
coroutine wrapper.

Cheers,
Nick.

P.S. Making that point reminded me that I still haven't got around to
updating those docs to also include examples of how to do it *right*:
https://bugs.python.org/issue30578

-- 
Nick Coghlan   |   ncoghlan at gmail.com   |   Brisbane, Australia