Oppiskele Async Context Managers | Tasks and Scheduling

Pyyhkäise näyttääksesi valikon

You already use context managers in regular Python: with open(file) as f ensures the file is closed even if an error occurs. Async context managers work the same way – but their setup and teardown can themselves be awaitable operations.

The Problem They Solve

Opening an HTTP session, a database connection, or a file over a network involves I/O. A regular with statement can't await anything inside __enter__ and __exit__. Async context managers add __aenter__ and __aexit__ – async versions of those methods.

`async with` Syntax

Use async with exactly like with, but inside an async def function:


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import asyncio
import httpx
import nest_asyncio

nest_asyncio.apply()

# Opening an HTTP session as an async context manager
async def fetch_user(user_id):
    async with httpx.AsyncClient() as client:  # __aenter__ called here
        response = await client.get(
            f"https://jsonplaceholder.typicode.com/users/{user_id}"
        )
        data = response.json()
        print(f"{data['name']} – {data['email']}")
    # __aexit__ called here – session closed automatically

asyncio.run(fetch_user(1))

The httpx.AsyncClient() is an async context manager. When the async with block exits – whether normally or due to an exception – the client closes all open connections.

Writing Your Own Async Context Manager

Use @asynccontextmanager from contextlib to define one with a generator:


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import asyncio
from contextlib import asynccontextmanager
import nest_asyncio

nest_asyncio.apply()

# Simulating a database connection lifecycle
@asynccontextmanager
async def db_connection(db_name):
    print(f"Opening connection to {db_name}")
    await asyncio.sleep(0.1)  # Simulating async connection setup
    try:
        yield db_name  # The value bound to the `as` variable
    finally:
        print(f"Closing connection to {db_name}")
        await asyncio.sleep(0.1)  # Simulating async cleanup

async def main():
    async with db_connection("analytics_db") as db:
        print(f"Running query on {db}")
        await asyncio.sleep(0.2)  # Simulating a query

asyncio.run(main())

The yield splits setup (before) from teardown (after). The finally block guarantees cleanup even if the body raises an exception.

Nesting Async Context Managers

Multiple async with statements can be combined on one line:


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import asyncio
import httpx
from contextlib import asynccontextmanager
import nest_asyncio

nest_asyncio.apply()

# Simulating a cache connection
@asynccontextmanager
async def cache_connection():
    print("Opening cache")
    await asyncio.sleep(0.05)
    try:
        yield "cache"
    finally:
        print("Closing cache")

async def main():
    async with httpx.AsyncClient() as client, cache_connection() as cache:
        response = await client.get(
            "https://jsonplaceholder.typicode.com/posts/1"
        )
        data = response.json()
        print(f"Fetched: {data['title']}")
        print(f"Would cache in: {cache}")

asyncio.run(main())

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Osio 2. Luku 5

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Osio 2. Luku 5

Async Context Managers

The Problem They Solve

async with Syntax

Writing Your Own Async Context Manager

Nesting Async Context Managers

`async with` Syntax