One Request ID Through Many Coroutines

An asynchronous request handler may call several coroutines before it sends a response. A request ID is useful in each one, especially when writing logs. Python’s context variables let code read a value from its current context without adding an ID argument to every call.

The value belongs to a context

Declare one ContextVar at module level. At the start of a request, call set() with that request’s ID. Code running in the current context can then call get(). set() returns a token that reset() can use to restore the previous value. Python recommends module-level declarations because contexts retain strong references to context variables. These behaviors are documented in contextvars.

The distinction matters when requests overlap. An asyncio task runs with a context. By default, creating a task copies the current context. Create child tasks after setting the request ID, and each starts with that binding. Another request can set a different ID in its own context.

The ID reaches child coroutines

import asyncio
from contextvars import ContextVar

request_id = ContextVar("request_id")

async def read_id():
    await asyncio.sleep(0)
    return request_id.get()

async def handle_request(identifier):
    token = request_id.set(identifier)
    try:
        first = asyncio.create_task(read_id())
        second = asyncio.create_task(read_id())
        return await asyncio.gather(first, second)
    finally:
        request_id.reset(token)

Both child tasks are created while identifier is set, so both can read it. The finally block restores the handler’s previous binding even if the awaited work raises an exception. The example follows the documented behavior of ContextVar tokens and asyncio task creation.

Task creation is the point to watch. A task receives a copy of the current context when it is created, unless a context is supplied explicitly. Setting a different ID in the parent afterward does not replace the binding already copied into that task. ContextVar.get() reads the value in the context where the code runs. Python documents both sides of that behavior.

What to do

1. Define the request ID variable once at module level.
2. Set it when the request starts, before creating child tasks.
3. Read it where a log entry or diagnostic message needs the ID.
4. Keep the token and reset it in a finally block.
5. Check where your application creates tasks. That moment determines which context each task copies.