I am learning asyncio callbacks. My task is- I have a message dict, message codes are keys, message texts are values. In coro main
I have to create a number of asynchronous tasks (in my case 3 tasks), each task wraps a coro which prints one message. Also I have to add a callback to each task. Callback must print a code associated with a message printed by the coroutine wrapped by the task. The question is- how to pass code to callback? The staright solution is to add name to each task with the value of a code, but I dont want to go this way. I decided to use ContextVar
for this purpose. So I create a global context variable, set()
the value to the variable equal to code. Then I try to get()
the context variable value from a callback but receive an Exception LookupError: <ContextVar name='msg_code' at 0x000001C596D94F40>
. That's my code:
import asyncio
from contextvars import ContextVar
msg_dict = {
'code1': 'msg1 by code1',
'code2': 'msg2 by code2',
'code3': 'msg3 by code3'
}
msg_code = ContextVar('msg_code')
async def print_msg(code):
await asyncio.sleep(0.5)
msg_code.set(code)
print(f'Message: {msg_dict[code]}')
def callback_code(*args):
code = msg_code.get()
print(f'Code: {code}')
async def main():
tasks = [asyncio.create_task(print_msg(code)) for code in msg_dict.keys()]
[task.add_done_callback(callback_code) for task in tasks]
await asyncio.gather(*tasks)
asyncio.run(main())
I found that add_done_callback()
also has keyword argument context=
but I can't find any examples of how to pass task's context to a callback.
No tricks are needed, just specify the context.
async def main():
tasks = []
for code in msg_dict:
ctx = copy_context() # note: import copy_context from contexvars
task = asyncio.create_task(print_msg(code), context=ctx)
task.add_done_callback(callback_code, context=ctx)
tasks.append(task)
await asyncio.gather(*tasks)