New submission from Andrew Svetlov <[email protected]>:
Now asyncio creates a new context copy on task creation.
It is the perfect behavior *by default* and should stay as is.
However, sometimes passing an explicit context into a task and using back the
context modified by task code is desired.
The most obvious use-case is testing: both unittest and pytest have multi-phase
test initialization (asyncSetUp() methods and async fixtures correspondingly).
If asyncSetUp() updates context variable -- test code expects to see this
change.
Currently, unittest runs the setup-test-cleanup chain in a single task and uses
an internal queue to push a new coroutine for execution and get results back.
It works but is cumbersome.
Another solution is to create a task per test execution step and wrap the task
creation with Context.run(). The problem is in getting the updated context
back. A task creates a context copy on starting, thus the modified context is
stored in the task internal attribute only. To get it back a trampoline async
function should be used, e.g.
async def wrapper(coro):
try:
return await coro
finally:
context = contextvars.copy_context()
# store the context copy somewhere
Again, it looks more complicated as it should be.
The proposal is:
1. Add 'context' keyword-only argument to asyncio.create_task() and
loop.create_task().
2. Use this context if explicitly passed, create a copy of the current context
otherwise.
The proposal is backward-compatible. Low-level API (call_soon(), call_later()
etc.) already accept 'context' argument.
The attached PR demonstrates how the proposed API simplifies
unittest.IsolatedAsyncioTestCase internals.
----------
components: asyncio
messages: 414988
nosy: asvetlov, yselivanov
priority: normal
severity: normal
status: open
title: Accept explicit contextvars.Context in asyncio create_task() API
type: enhancement
versions: Python 3.11
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Python tracker <[email protected]>
<https://bugs.python.org/issue46994>
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