Python 3.4+Async programmingterm

asyncio.BoundedSemaphore

synchronization

A semaphore that never lets its counter rise above the initial value: an extra release() raises ValueError instead of silently letting one more task through.

Syntax

sem = asyncio.BoundedSemaphore(value=1)

Used correctly it behaves exactly like asyncio.Semaphore; the difference is the error: Semaphore(2) after a stray release() admits three tasks, whereas BoundedSemaphore(2) immediately reports 'released too many times'. So for 'at most N at once' it is the sensible default — a counting bug shows up at once, not a week later under load.

Examples

import asyncio
async def main():
    sem = asyncio.BoundedSemaphore(1)
    async with sem:
        await asyncio.sleep(0.01)
    try:
        sem.release()  # лишний release
    except ValueError as e:
        print(e)  # → BoundedSemaphore released too many times
asyncio.run(main())

asyncio.BoundedSemaphore

Syntax

Examples

See also