asyncio-loop-local

asyncio-loop-local storage, singletons and deferred aexit's. Init that pool once and reuse it!

why have loop-local things

Suppose you have some async context manager, like aiohttp.ClientSession:

async with aiohttp.ClientSession() as session:
    async with session.get('http://httpbin.org/get') as resp:
        print(await resp.text())

But the docs tell you: "don't create a session per request". So, you need to reuse it somehow, and this means you need to init it somewhere and then either pass it around all the code that might need it, or... use global variables? There must be a better way.

sticky_singleton_acm

The all-in-one solution to this is:

_ClientSession = asyncio_loop_local.sticky_singleton_acm(ClientSession)

...

async _ClientSession() as session:
    async with session.get('http://httpbin.org/get') as resp:
        print(await resp.text())

The wrapper gives you

• the same ClientSession for every combination of parameters passed to it (singleton)
• that doesn't __aexit__ when the block is over, only when the async event loop is closed (sticky ACM)

enter_once + singleton

Those willing to save up a level of indentation (after all, de-denting is a no-op as __aexit__'ing effect is deferred anyway) might be interested in combining two simpler primitives to the same effect:

_ClientSession = asyncio_loop_local.singleton(ClientSession)

...

session = await asyncio_loop_local.enter_once(_ClientSession())
async with session.get('http://httpbin.org/get') as resp:
    print(await resp.text())

singleton

Decorate your callable that generates something, and the returned value will be cached per async loop (and per passed parameters).

_ClientSession = asyncio_loop_local.singleton(ClientSession)

_ClientSession() is _ClientSession()  # as long you're in the same event loop

enter_once

Invoke __anter__ of an asynchronous context manager now, defer __aexit__ to the end of the event loop. Repeated attempts to enter_once on the same object will do nothing.

session = await asyncio_loop_local.enter_once(ClientSession())
...
# __aexit__ will be executed at the end of the event loop

sticky_acm

Wrap an asynchronous context manager. __anter__ just proxies the original context manager's __aenter__, __aexit__ does nothing and is deferred to end of the event loop. It's like enter_once, but in a form to use with async with.

cs = asyncio_loop_local.sticky_acm(ClientSession())
async with cs:  # __aenter__'s
    ...
# __aexit__ will be executed at the end of the event loop

enter

Invoke __anter__ of an asynchronous context manager now, defer __aexit__ to the end of the event loop.

session = await asyncio_loop_local.enter_once(ClientSession())
...
# __aexit__ will be executed at the end of the event loop

storage

Async-loop-local storage. Like thread-local, but loop-local. All the hacks above are built up on it.

alls = asyncio_loop_local._storage.storage()  # gives a loop-associated dict
alls['purpose'] = ('whatever', {'you': 'want'})

If you wanna use locking, consider this:

class LockingDict(asyncio.Lock, dict): pass
alls['careful'] = LockingDict()
with alls['careful'] as d:
   d['a'] = 'b'

_atexit

There's also an async-loop-local atexit hook implementation. It's private so far until some interest is expressed.