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). 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.)
On Tuesday, May 5, 2015 7:01 AM, Koos Zevenhoven <email@example.com> wrote:
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 :)
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.
This is simplified version; for instance, as "awaitables", I consider only async function objects here. I describe the idea in three parts:
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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