Async context managers: async with и @asynccontextmanager

Async context managers управляют ресурсами в async коде — соединениями, файлами, транзакциями.

aenter и aexit

class AsyncDB:
    async def __aenter__(self):
        self.conn = await connect("postgresql://localhost/mydb")
        return self.conn

    async def __aexit__(self, exc_type, exc_val, exc_tb):
        await self.conn.close()
        return False  # не подавляем исключения

async def main():
    async with AsyncDB() as conn:
        result = await conn.fetch("SELECT * FROM users")

@asynccontextmanager — проще чем класс

from contextlib import asynccontextmanager
import httpx

@asynccontextmanager
async def managed_client(base_url: str):
    client = httpx.AsyncClient(base_url=base_url, timeout=30.0)
    try:
        yield client
    finally:
        await client.aclose()

async def main():
    async with managed_client("https://api.example.com") as client:
        r = await client.get("/users")

aiofiles — async работа с файлами

import aiofiles

async def save_result(filename: str, data: str):
    async with aiofiles.open(filename, "w", encoding="utf-8") as f:
        await f.write(data)

async def read_file(filename: str) -> str:
    async with aiofiles.open(filename, "r", encoding="utf-8") as f:
        return await f.read()

# Обработка по строкам — не грузим всё в память
async def process_large_file(filename: str):
    async with aiofiles.open(filename) as f:
        async for line in f:
            await process_line(line.strip())

Вложенные context managers

async def main():
    async with (
        httpx.AsyncClient() as client,
        aiofiles.open("results.json", "w") as f,
    ):
        for url in urls:
            data = await client.get(url)
            await f.write(data.text + "\n")

Python 3.10+ поддерживает скобочный синтаксис для нескольких async with — без бэкслешей.

Паттерн: async context manager для транзакций

@asynccontextmanager
async def transaction(conn):
    await conn.execute("BEGIN")
    try:
        yield conn
        await conn.execute("COMMIT")
    except Exception:
        await conn.execute("ROLLBACK")
        raise