Semaphore

Learn concurrency limits with asyncio.Semaphore.

Semaphore

asyncio.Semaphore limits how many coroutines run at once. It generalizes Lock: it allows n coroutines; the rest wait.

sem = asyncio.Semaphore(3)

async with sem:
    # at most 3 coroutines here
    await fetch()

This is useful to avoid overloading an external service. To limit API requests to 3 at a time, use Semaphore(3).

The semaphore value decreases on each acquire and increases on each release. async with does both automatically.

Limiting and resource control

Semaphore is ideal for controlling external resource use. Instead of hundreds of API requests at once, with Semaphore(10) you progress in groups of 10.

This balances speed and quota limits: not too slow, but the service is not overloaded either. The value is found by trial and error; typical values are 5-20.

Lock is effectively Semaphore(1); a single-permit exclusive access.

Exercise Limited access with Semaphore

Start with import asyncio. Define sem = asyncio.Semaphore(2). The function async def fetch(n) returns n inside async with sem:. Inside async def main() call await asyncio.gather(fetch(1), fetch(2), fetch(3)) and return it. Add print(asyncio.run(main())).

Show example solution

import asyncio

sem = asyncio.Semaphore(2)

async def fetch(n):
    async with sem:
        return n

async def main():
    return await asyncio.gather(fetch(1), fetch(2), fetch(3))

print(asyncio.run(main()))

Test yourself

Match the Semaphore concepts

Semaphore(3)

Three permits
async with sem

Acquire and release
acquire

Asks permit
release

Releases permit
What is the relation between Lock and Semaphore(1)?
- They are completely different
- Lock is effectively Semaphore(1)
- Semaphore(1) is stronger than Lock
- Lock equals Semaphore(2)
Show answer

Lock is effectively Semaphore(1) — Lock is a single-permit Semaphore; that is, it behaves like Semaphore(1).

Define a Semaphore

import asyncio

sem = asyncio.____(3)

Show answer

import asyncio

sem = asyncio.Semaphore(3)

Next lesson: Summary - Asyncio

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