[FIXED] asyncio.Semaphore cannot be used with the async with as syntax

Issue

I am a Python programming enthusiast, and recently encountered the issue demonstrated in the code below while learning about the usage of asyncio.

import asyncio

async def a():
    sem = asyncio.Semaphore(2)
    async with sem:
        print(sem)

async def b():
    async with asyncio.Semaphore(2) as sem:
        print(sem)

if __name__ == "__main__":
    asyncio.run(a())
    asyncio.run(b())

"""
output:
<asyncio.locks.Semaphore object at 0x7f28e4eaca60 [unlocked, value:1]>
None
"""

I am curious as to why this is happening. Is it because asyncio.Semaphore objects cannot be used with the async with as syntax?

The Python version I am using is Python 3.9 running on AlmaLinux, I have looked up the syntax requirements for the 'as' keyword in Python and tried searching for answers on Stack Overflow, but I haven't been able to find out why this is happening. I would appreciate it if someone could help me clear up this confusion, and any reply are welcome. Thanks.

Solution

Consider this example:

import asyncio

async def a():
    sem = asyncio.Semaphore(2)
    async with sem:
        print(sem)

async def b():
    async with (actual_semaphore:=asyncio.Semaphore(2)) as value_returned_from_aenter:
        print(f'{value_returned_from_aenter=}')
        print(f'{actual_semaphore=}')

if __name__ == "__main__":
    asyncio.run(a())
    asyncio.run(b())

Prints:

<asyncio.locks.Semaphore object at 0x7f41067e69e0 [unlocked, value:1]>
value_returned_from_aenter=None
actual_semaphore=<asyncio.locks.Semaphore object at 0x7f41067e69e0 [unlocked, value:1]>

---

You see None because the vale returned from __aenter__ is None.

Source: https://github.com/python/cpython/blob/main/Lib/asyncio/locks.py#L12

Answered By - Andrej Kesely