A simple asyncio wrapper attempting to look like Trio
A simple asyncio wrapper attempting to look like Trio
When going deeper and deeper with asyncio, and managing a lot of tasks in parallel, you notice that on top of having a lot to deal with to keep an eye on all your task, you also end up always doing the same kind of boiler plate, and the code can become easily twisted and unreadable.
Trio is there to make asynchronous programming easy and more "for humans".
Traio (as kind of Trio on top of Asyncio) let you use asyncio with a little of the philosophy of Trio, mostly the Nursery concept (called Scope here). It also synthesize the most common pattern we are using for handling async tasks in a sane way.
This is not a replacement for Trio: if you do a full Trio-like project, just use Trio! It's just truly awesome, but in some cases you get stuck with asyncio, but still want to have a code you can read and manage...
Because we run on top of asyncio, we are quite limited in how we can handle cancellation, scopes, and coroutines. This is just on top of asyncio, this is not an alternative! But on the good side, you can mix this with regular asyncio code, which can look appealing.
The main way to use the Scope is (like in Trio) as a context manager
import asyncio from traio import Scope async def fetch_url(x): # Do something long await asyncio.sleep(3) async def main(): async with Scope(timeout=10) as scope: for i in range(10): scope.spawn(fetch_url(i))
Scope.spawn method, called on an awaitable thing, will spawn a task and
register it on the scope. An equivalent exist using the
<< operator (see next
When reaching the end of the context block, the code will block until:
- all tasks are done
- or the timeout is over
- or the scope get's cancelled.
You can also use the Scope without context manager:
import asyncio from traio import Scope async def fetch_url(x): # Do something long await asyncio.sleep(3) async def main(): # Equivalent to previous example scope = Scope(timeout=10) for i in range(10): scope << fetch_url(i) scope.finalize() await scope
Awaiting a scope will block until the scope is fully complete: all active tasks
have finished or the scope was cancelled. But unless
scope.finalize() is called,
a scope will note stop on the last task being complete, only on cancellation.
finalize method is called automatically when used as a context manager.
Names and logger
If you went deep enough in asyncio mysteries, you know that tracing code is (for now) kind of a nightmare... For that reason, Scope as well as tasks can be instantiated with a name. The Scope can also take a logger which will be used for tracing most of the calls and task life cycle, mostly with debug level.
Scope.spawn can be called with different parameters with interesting
bubbleboolean parameter controls task error bubbling. A task will bubble by default. This means that an error in the task will cause the task to stop (of course), but the scope will be cancelled as well and raise the given error. This is the default behavior. But it can be useful in some cases not to do that, and just ignore a task. Not that if you await manually a task, or add a done callback, this cancels bubbling automatically: if you take the pain of waiting for a task, it's not to get all the rest cancelled!
import asyncio from traio import Scope async def fetch_url(): # Do something long await asyncio.sleep(10) async def trivial(): await asyncio.sleep(0.01) raise ValueError('not interesting') async def main(): async with Scope(timeout=0.5) as n: # This will try to run for 10 seconds n << fetch_url() # This will run a bit then raise (but not 'bubble') n.spawn(trivial(), bubble=False) # Eventually after 0.5 seconds the Scope times out and # gives a TImeoutError
- A task can be marked as
master, in that case the scope will die with the task when done. This is typically useful when you have one main task to be performed and other background ones, which have no meaning if the main one stops.
import asyncio from traio import Scope async def fetch_url(): # Do something long await asyncio.sleep(10) async def trivial(): await asyncio.sleep(0.01) async def main(): async with Scope(timeout=0.5) as n: # This will try to run for 10 seconds n << fetch_url() # This will run a bit then scope gets cancelled when it ends n.spawn(trivial(), master=True)
- A task by default is
awaited, which means the scope will wait for it to finish during finalisation stage, before exiting. It is possible to mark some tasks as not
awaitedif you want a task running, but not so essential that it should prevent cancellation. Typically, a background job which has no meaning alone.
import asyncio from traio import Scope async def background(): while True: # Do something periodic and sleep await asyncio.sleep(1) async def job(): # Do the real work await asyncio.sleep(10) async def main(): async with Scope() as n: # Spawn a background job n.spawn(background(), awaited=False) # Do what you have to do. n << job() # At this point, job is done and background task was cancelled
Note you can combine all flags; If bubble is False and master is True, the scope will exit silently when the task is done, even with an exception, for example.
That's one of the most exciting ones: you can spawn a sub-scope from the original one, which will follow the same rules as any Scope but will as well die if the parent is cancelled!
import asyncio from traio import Scope async def fetch_url(): # Do something long await asyncio.sleep(10) async def main(): async with Scope(timeout=0.2) as parent: async with parent.fork(timeout=0.5) as inner: # This will try to run for 10 seconds inner << fetch_url() # Here the outer Scope will timeout first and will cancel the inner one!
Running synchronous code with executors
It is possible to run synchronous code from a scope using the
loop.run_in_executor method; this would return a coroutine which
you can spawn as usual. But beware of using that method! The same way
interrupts handling is a mess in asyncio, using thread from asyncio is tricky.
Most OS don't support cancelling running threads, so does Python.
As such, once your executor code is running, the scope has no way to
actually stop it; if you cancel the scope, the corresponding future will be
cancelled but the task will continue running if not yet finished, resulting in
various "fun" situations if that task has side effects...
So: yes, you can use this, but at your own risks!
import asyncio import time from traio import Scope def fetch_url(): # Do something long, sync! time.sleep(10) async def main(): async with Scope(timeout=0.2) as scope: scope << asyncio.get_event_loop().run_in_executor(None, fetch_url)
Getting current Scope
async def handler(client): # This runs in the scope `main_scope` # Create a subscope of the current one async with Scope.get_current().fork() as s: # Here we are in the scope of `s`, as well as spawned tasks s << do_something(client) s << do_something_else(client) async def server(main_scope): # This runs in the scope `main_scope` client = await new_connection() main_scope.spawn(handler(client)) async def main(): async with Scope() as main_scope: main_scope << server(main_scope) main_scope << do_something_else()
This is beta. We are not going to change the API (much) anymore.
- write more examples
- extend the API:
- new kinds of tasks to investigate, for example executors (although we have no way to stop a thread executor...)
- play more with the real Trio and get a better feeling of it
- get some user feedback if possible!
You may like as well Ayo, discovered in parallel of writting this lib!
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