asyncio context propagation in Python 3.5/3.6

[a-feld]

The contextvars approach to context propagation requires copying current context on asyncio task creation. This is done internally in asyncio for Python 3.7; however, on Python <3.7 context is not propagated automatically.

Example:

import asyncio
import contextvars

current_task = contextvars.ContextVar("current_task")


async def foo(foo_started, bar_done):
    current_task.set("foo")
    foo_started.set()
    await bar_done.wait()
    print("foo task:", current_task.get())


async def bar(foo_started, bar_done):
    await foo_started.wait()
    current_task.set("bar")
    bar_done.set()


async def main():
    foo_started = asyncio.Event()
    bar_done = asyncio.Event()

    await asyncio.gather(
        foo(foo_started, bar_done),
        bar(foo_started, bar_done),
    )


loop = asyncio.get_event_loop()
loop.run_until_complete(main())

In Python 3.7, the code above will print foo task: foo
In Python 3.5 and 3.6, the code above will print foo task: bar

[toumorokoshi]

I've used https://github.com/Skyscanner/aiotask-context before this functionality existed natively.

[condorcet]

Hi there! The alternative is to use contextvars backport -- aiocontextvars https://github.com/fantix/aiocontextvars/
But it has some limitations:

• It doesn't work with python < 3.5.
• It doesn't support context propagation for methods like call_soon (but we can achieve this feature by the PR Patch call_soon and friends. fantix/aiocontextvars#66) so behavior in python 3.5/3.6 and 3.7 will be different. You can see tests about call_soon in this PR to opentracing AsyncioScopeManager based on contextvars and supporting Tornado 6 opentracing/opentracing-python#118

As I understand aiotask-context has the same problems especially the last one. Big advantage of aiocontextvars is drop-in replacement original context without code modification.

[mauriciovasquezbernal]

@codeboten was it solved by #395?