async with lock:

Concurrency & Async

Meaning

Acquires an asyncio lock for exclusive access within an async context manager, automatically releasing it when the block exits. Addresses race conditions where multiple coroutines access shared mutable state concurrently. Used whenever coordinated exclusive access to a shared resource is needed in an asynchronous program.

Primary Function

Concurrency

Communicative Purpose

Prevents concurrent coroutines from accessing shared state simultaneously

Pattern

async with lock:

Core Structure

async with ...:

Função primária

Concurrency

Propósito comunicativo

Prevents concurrent coroutines from accessing shared state simultaneously

Situações de gatilho

Async web servers: protecting shared in-memory caches during concurrent request handling, Data pipelines: serializing writes to a shared async queue or file descriptor, Background tasks: coordinating access to a limited connection pool

Contextos

asyncio, aiohttp, FastAPI, anyio, Python async applications

Padrão

async with lock:

Estrutura central

async with ...:

Slots de substituição

lock: asyncio.Lock or compatible async context manager object

Colocados típicos

asyncio.Lock()
asyncio.Event()
asyncio.Semaphore()
asyncio.Condition()
async with
await

Substituições comuns

await lock.acquire() / lock.release() — manual acquisition requires explicit release in finally block
more error-prone
threading.Lock — wrong for async code
blocks the event loop

Erros comuns

Using threading.Lock instead of asyncio.Lock — blocks the entire event loop thread, freezing all coroutines, Forgetting async keyword before with — SyntaxError or uses synchronous context manager, Acquiring lock without context manager and forgetting lock.release() in finally — lock never released on exception, causing deadlock, Creating a new Lock inside the async with block instead of sharing one — each coroutine gets its own lock, no mutual exclusion achieved

Similar / contraste

asyncio.Semaphore — allows N concurrent accessors instead of exclusive one, asyncio.Event — signals readiness rather than exclusive access, threading.Lock — synchronous lock that blocks OS thread, not awaitable

Interferências

Coming from threading: using threading.Lock in async code blocks the event loop — must use asyncio.Lock instead, Coming from Go: expecting lock to be implicitly released on scope exit — Python requires explicit context manager or manual release, Coming from JavaScript: expecting all async primitives to be promise-based — Python asyncio locks are coroutine-based and require await

Família do chunk

asyncio.Lock
asyncio.Event
asyncio.Semaphore
asyncio.Condition
asyncio.Barrier
async with

Nuance

Do not use when the critical section is purely I/O-bound with no shared mutable state — unnecessary serialization reduces throughput. Acquiring an asyncio.Lock is O(1) but contention causes coroutines to queue, increasing latency under load. The lock is not reentrant — re-acquiring the same lock from the same coroutine causes deadlock.

Efeito pragmático

Eliminates data races on shared state in async programs without blocking the event loop, ensuring correctness while maintaining concurrency for non-contending operations

Dica de memória

Like a bathroom lock in a shared office — only one person inside at a time, and the lock auto-releases when you leave even if you faint (exception).

Nota

asyncio.Lock is a cooperative lock — it only prevents other coroutines from acquiring it, not other threads. For thread-safety across threads, use asyncio.Lock with an executor or threading primitives.

Upgrade path

asyncio.Semaphore for bounded concurrency, asyncio.Condition for wait/notify patterns

Frequência: HighFormulaicidade: Semi-fixedTipo de construção: clausePrioridade de aquisição: Active recallPrioridade de output: OutputTag de espaçamento: Short-term

Examples

import asyncio counter = 0 lock = asyncio.Lock() async def increment(): global counter async with lock: current = counter await asyncio.sleep(0.001) # simulate I/O counter = current + 1 async def main(): await asyncio.gather(*[increment() for _ in range(100)]) print(f"counter = {counter}") # reliably 100 asyncio.run(main())

import asyncio import json from pathlib import Path file_lock = asyncio.Lock() CONFIG_PATH = Path("config.json") async def update_config(key: str, value: str) -> None: async with file_lock: data = json.loads(CONFIG_PATH.read_text()) data[key] = value await asyncio.sleep(0.01) # simulate slow write CONFIG_PATH.write_text(json.dumps(data, indent=2)) async def main(): await asyncio.gather( update_config("host", "localhost"), update_config("port", "8080"), ) asyncio.run(main())

import asyncio import aiohttp pool_lock = asyncio.Lock() active_connections = 0 MAX_CONNECTIONS = 5 async def fetch(session: aiohttp.ClientSession, url: str) -> str: global active_connections async with pool_lock: while active_connections >= MAX_CONNECTIONS: await asyncio.sleep(0.1) active_connections += 1 try: async with session.get(url) as response: return await response.text() finally: async with pool_lock: active_connections -= 1 async def main(): async with aiohttp.ClientSession() as session: urls = ["https://httpbin.org/get" for _ in range(10)] results = await asyncio.gather( *[fetch(session, url) for url in urls] ) asyncio.run(main())