Python 3.7+contextvarsterm

contextvars.ContextVar

context variables

A context variable: each asyncio task and each thread sees its own value; the replacement for threading.local in async code.

Syntax

var = contextvars.ContextVar(name, *, default=...)

In asyncio all tasks run on one thread, so a threading.local is shared and values overwrite each other; a ContextVar is copied into each task by create_task() and does not leak between them. Create it once at module level, not inside a function. get() with no value and no default raises LookupError. The default is shared by every context: a mutable list there is common to all.

Examples

import asyncio, contextvars, threading
local = threading.local()
request_id = contextvars.ContextVar('request_id')
async def handle(n):
    local.n = n
    request_id.set(n)
    await asyncio.sleep(0.01 * (3 - n))
    return local.n, request_id.get()
async def main():
    return await asyncio.gather(*(handle(i) for i in range(3)))
print(asyncio.run(main()))  # → [(2, 0), (2, 1), (2, 2)] — local перетёрт

import contextvars
user = contextvars.ContextVar('user')
print(user.get('гость'))  # → гость
try:
    user.get()
except LookupError:
    print('значения нет')  # → значения нет

contextvars.ContextVar

Syntax

Examples

See also