Please don't use ContextVar.reset in async_generator.

README.md

Please don't use ContextVar.reset in async_generator.

Important rule: Different coroutines own different contexts.

• The example1.py runs one coroutine with async_generator, the async_generator.__anext__ method runs in same context and thus it works find.
• The example2.py produces two coroutines with the same async_generator, the async_generator.__anext__ method runs in different contexts and thus it raises an ValueError exception.

Tips for async_generator with ContextVar

1. don't use reset in async_generator.
2. need to ensure the context from gen.__anext__() being applied on the procedure of per loop.

async def process_per_loop():
    await gen.__anext__()
    await proceduer()  # do something with same gen.__anext__() context
    asyncio.ensure_future(coro())  # do something with copied gen.__anext__() context

3. best way is using async-for statement.

async for val in gen:
    await proceduer()  # do something with same gen.__anext__() context
    asyncio.ensure_future(coro())  # do something with copied gen.__anext__() context

example1.py

import asyncio

from contextvars import ContextVar

var = ContextVar('bar')

async def gen():
    token = var.set(1)
    yield
    var.reset(token)


coro_gen = gen()
await coro_gen.__anext__()
try:
    await coro_gen.__anext__()
except StopAsyncIteration:
    pass

example2.py

import asyncio

from contextvars import ContextVar

var = ContextVar('bar')

async def gen():
    token = var.set(1)
    yield
    var.reset(token)


coro_gen = gen()
await asyncio.ensure_future(coro_gen.__anext__())
try:
    await coro_gen.__anext__()
except StopAsyncIteration:
    pass
  
# ValueError: <Token var=<ContextVar name='bar' at 0x10ed92130> at 0x10de9c100> was created in a different Context