Hi Guido and Andrew,
Thank you for your prompt responses!
On 5.5.2015 21:48, Guido van Rossum wrote:
Quick notes:
And since there is asyncio in the standard library, I would assume there typically is no reason to do that either(?) However, as a side effect of my proposal, there would still be a way to use an if statement to pick the right async code to match the framework, along with matching the non-async version :).
Speaking of side effects, I think the same "__async__" variable might also naturally provide this:
https://mail.python.org/pipermail/python-ideas/2015-April/033152.html
By the way, if I understand your first note, it might be the same as my "The Y and L ends need to be compatible with each other for the code to work." Sorry about the terminology. I hope the explanations of Y and L are somewhat understandable.
This is what is keeping me from using asyncio. Ignoring performance overhead, if in any synchronous script (or interactive prompt or ipython notebook) all calls to my library would look like that, I will happily use my 2.7 version that uses threads. Well, I admit that the part about "happily" is not completely true in my case.
Instead, I would be quite happy typing "await <function_call>", since awaiting the function call (to finish/return a value) is exactly what I would be doing, regardless of whether there is an event loop or not.
Regarding my proposal, there would still be a way for libraries to provide this functionality, if desired :).
Please see also the comments below.
On Tue, May 5, 2015 at 11:00 AM, Andrew Barnert via Python-ideas
<python-ideas@python.org mailto:python-ideas@python.org> wrote:
It seems like it might be a lot easier to approach this from the
other end: Is it possible to write a decorator that takes an async
coroutine function, strips out all the awaits, and returns a
regular sync function? If so, all you need to do is write
everything as async, and then users can "from spam import sync as
spam" or "from spam import async as spam" (where async just
imports all the real functions, while sync imports them and calls
the decorator on all of them).
Interesting idea. If this is possible, it would solve part of the issue, but the "Y end" (sorry) of the chain may still need to be done by hand.
>
That also avoids the need to have all the
looking up the event
loop, switching between different code branches, etc. inside every
function at runtime. (Not that it matters for the performance of
sleep(1), but it might matter for the performance of other
functions—and, more importantly, it might make the implementation
of those functions simpler and easier to debug through.)
This could indeed save some if statements at runtime.
Note that the if statements would not be inside every function, but only in the ones that do the actual IO. For instance, some 3rd-party library might use wrappers around socket send and socket recv to choose between sync and async versions, and that might be all the IO it needs to build several layers of async code. Even better, had someone taken the time to provide these if statements inside the standard library, the whole 3rd-party async library would just magically work also in synchronous code :).
Best regards, Koos
>
On Tuesday, May 5, 2015 7:01 AM, Koos
Zevenhoven
<koos.zevenhoven@aalto.fi <mailto:koos.zevenhoven@aalto.fi>> wrote:
Hi all!
I am excited about seeing what's going on with asyncio and
PEP492 etc. I
really like that Python is becoming more suitable for the
increasing
amount of async code and that the distinction between async
functions
and generators is increasing.
In addition, however, I would also like to see the async
functions and
methods come even closer to regular functions and methods.
This is
something that is keeping me from using asyncio at the moment
even if I
would like to. Below I'll try to explain what and why, and a
little bit
of how. If it is not clear, please ask :)
Motivation:
One of the best things about asyncio and coroutines/async
functions is
that you can write asynchronous code as if it were
synchronous, the
difference in many places being just the use of "await"
("yield from")
when calling something that may end up doing IO (somewhere
down the
function call chain) and that the code is run from an event loop.
When writing a package that does IO, you have the option to
make it
either synchronous or asynchronous. Regardless of the choice,
the code
will look roughly the same. But what if you want to be able to
do both?
Should you maintain two versions, one with "async" and "await"
everywhere and one without?
Besides the keywords "async" and "await", async code of course
differs
from synchronous code by the functions/coroutines that are
used for IO
at the end of the function call chain. Here, I mean the end
(close to)
where the "yield" expressions are hidden in the async
versions. At the
other end of the calling chain, async code needs the event
loop and
associated framework (almost always asyncio?) which hides all
the async
scheduling fanciness etc. I'm not sure about the terminology,
but I will
use "L end" and "Y end" to refer to the two ends here. (L for
event
Loop; Y for Yield)
The Y and L ends need to be compatible with each other for the
code to
work. While asyncio and the standard library might provide
both ends in
many cases, there can also be situations where a package would
want to
work with different combinations of L and Y end, or completely
without
an event loop, i.e. synchronously.
In a very simple example, one might want to wrap different
implementations of sleep() in a function that would pick the
right one
depending on the context. Perhaps something like this:
async def any_sleep(seconds):
if __async__.framework is None:
time.sleep(1)
elif __async__.framework is asyncio:
await asyncio.sleep(1)
else:
raise RuntimeError("Was called with an unsupported
async
framework.")
[You could of course replace sleep() with socket IO or
whatever, but
sleep is nice and simple. Also, a larger library would
probably have a
whole chain of async functions and methods before calling
something like
this]
But if await is only allowed inside "async def", then how can
any_sleep() be conveniently run in non-async code? Also, there is
nothing like __async__.framework. Below, I describe what I
think a
potential solution might look like.
Potential solution:
This is simplified version; for instance, as "awaitables", I
consider
only async function objects here. I describe the idea in three
parts:
(1) next(...):
Add a keyword argument "async_framework" (or whatever) to
next(...) with
a default value of None. When an async framework, typically
asyncio,
starts an async function object (coroutine) with a call to
next(...), it
would do something like next(coro, async_framework = asyncio).
Here,
asyncio could of course be replaced with any object that
identifies the
framework. This information would then be somehow attached to
the async
function object.
(2) __async__.framework or something similar:
Add something like __async__ that has an attribute such as
.framework
that allows the code inside the async function to access the
information
passed to next(...) by the framework (L end) using the keyword
argument
of next [see (1)].
(3) Generalized "await":
[When the world is ready:] Allow using "await" anywhere, not
just within
async functions. Inside async functions, the behavior of
"await" would
be the same as in PEP492, with the addition that it would somehow
propagate the __async__.framework value to the awaited coroutine.
Outside async functions, "await" would do roughly the same as
this function:
def await(async_func_obj):
try:
next(async_func_obj) # same as next(async_func_obj,
async_framework = None)
except StopIteration as si:
return si.value
raise RuntimeError("The function does not support
synchronous
execution")
(This function would, of course, work in Python 3.4, but it
would be
mostly useless because the async functions would not know that
they are
being called in a 'synchronous program'. IIUC, this *function*
would be
valid even with PEP492, but having this as a function would be
ugly in
the long run.)
Some random thoughts:
With this addition to Python, one could write libraries that
work both
async and non-async. When await is not inside async def, one
would
expect it to potentially do blocking IO, just like an await
inside async
def would suggest that there is a yield/suspend somewhere in
there.
For testing, I tried to see if there is a reasonable way to
make a hack
with __async__.framework that could be set by next(), but did
not find
an obvious way. For instance, coro.gi_frame.f_locals is
read-only, I
believe.
An alternative to this approach could be that await would
implicitly
start a temporary event loop for running the coroutine, but
how would it
know which event loop? This might also have a huge performance
overhead.
Relation to PEP492:
This of course still needs more thinking, but I wanted to post
it here
now in case there is desire to prepare for something like this
already
in PEP492. It is not completely clear if/how this would need
to affect
PEP492, but some things come to mind. For example, this could
potentially remove the need for __aenter__, __aiter__, etc. or
even
"async for" and "async with". If __aenter__ is defined as
"async def",
then a with statement would do an "await" on it, and the
context manager
would have __async__.framework (or whatever it would be called)
available, for determining what behavior is appropriate.
Was this clear enough to understand which problem(s) this
would be
solving and how? I'd be happy to hear about any thoughts on
this :).
Best regards,
Koos
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