await event.clear()

Concurrency & Async

Meaning

Resets an asyncio.Event's internal flag to False, causing subsequent calls to wait() to block until set() is called again. Used to re-arm an event for the next signaling cycle after a wait() has returned. Essential for implementing cyclic signaling patterns between coroutines.

Primary Function

Synchronization

Communicative Purpose

Ensures that subsequent wait() calls will block until the event is set again, enabling repeated signaling cycles.

Pattern

event.clear()

Core Structure

event.clear()

Função primária

Synchronization

Propósito comunicativo

Ensures that subsequent wait() calls will block until the event is set again, enabling repeated signaling cycles.

Situações de gatilho

Implementing producer-consumer cycles, resetting a signal after processing a batch, awaiting a repeated trigger in async loops.

Contextos

Asyncio-based Python code, network servers, async GUI event loops, any code using asyncio.Event for coordination.

Padrão

event.clear()

Estrutura central

event.clear()

Slots de substituição

event: asyncio.Event instance

Colocados típicos

asyncio.Event
set()
wait()

Substituições comuns

Reassigning a new Event object (wasteful
loses existing waiters)
using a boolean flag manually (error-prone
no coroutine integration)

Erros comuns

1. Awaiting clear() — it is a regular method returning None, not a coroutine; await event.clear() raises TypeError. 2. Calling clear() before wait() returns — may cause wait() to block again immediately. 3. Forgetting to clear after handling an event — subsequent wait() calls return instantly if the flag is still set.

Similar / contraste

event.set() (signals), event.wait() (waits without clearing); using asyncio.Lock.acquire()/release()

Interferências

Coming from threading.Event: clear() works identically but asyncio.Event.clear() must be called from the event loop thread; Coming from JavaScript: no direct equivalent — Promise-based patterns differ significantly.

Família do chunk

asyncio event primitives
synchronization

Nuance

1. Do not use clear() when you need a one-shot signal that stays set permanently. 2. clear() is O(1) and does not schedule any callbacks — it simply resets the internal flag. 3. If waiters are already blocked on wait(), clearing the flag does NOT unblock them; they remain blocked until set() is called again.

Efeito pragmático

Guards that subsequent wait() will block until the event is set again, enabling cyclic signaling

Dica de memória

Like turning off a light switch so the next person has to flip it on again.

Nota

clear() is a synchronous method returning None — it must NOT be awaited. Awaiting it raises TypeError: object NoneType can't be used in 'await' expression.

Upgrade path

Consider using asyncio.Condition for more complex wait/notify patterns

Frequência: MediumFormulaicidade: Semi-fixedTipo de construção: await method call on asyncio.Event instancePrioridade de aquisição: Active recallPrioridade de output: BothTag de espaçamento: Medium-term

Examples

import asyncio async def consumer(event, items): while True: await event.wait() while items: print(f"Consumed: {items.pop(0)}") event.clear() async def producer(event, items): for i in range(3): await asyncio.sleep(0.1) items.append(f"item-{i}") event.set() async def main(): event = asyncio.Event() items = [] await asyncio.gather(producer(event, items), consumer(event, items)) asyncio.run(main())

import asyncio async def monitor(event): for _ in range(3): await event.wait() print("Alert received, re-arming detector") event.clear() async def trigger(event): for i in range(3): await asyncio.sleep(0.2) print(f"Triggering alert {i}") event.set() async def main(): event = asyncio.Event() await asyncio.gather(trigger(event), monitor(event)) asyncio.run(main())

import asyncio async def handler(event, data): while True: await event.wait() print(f"Handling: {data['payload']}") data['payload'] = None event.clear() async def updater(event, data): for i in range(3): await asyncio.sleep(0.15) data['payload'] = f"batch-{i}" event.set() async def main(): event = asyncio.Event() data = {'payload': None} await asyncio.gather(updater(event, data), handler(event, data)) asyncio.run(main())