Aiohttp 3 13 5

aiohttp 3.13

Overview

aiohttp is a comprehensive async HTTP client/server framework for Python built on top of asyncio. It provides both an HTTP client and an HTTP server, supporting WebSockets, streaming, middleware, signals, connection pooling, and more. Current stable version is 3.13.5.

Key features:

• Full-featured async HTTP client with connection pooling and keep-alive
• Async HTTP server with pluggable routing, middlewares, and signals
• WebSocket support on both client and server sides
• Streaming request/response bodies via StreamReader
• Multipart reader/writer for file uploads and complex payloads
• Client middleware for request/response interception
• Server middleware and signal system
• Built-in test utilities (TestClient, TestServer, AioHTTPTestCase)
• Gunicorn worker support (aiohttp.GunicornWebWorker)

Dependencies: attrs, multidict, yarl. Optional: aiodns (faster DNS), Brotli or brotlicffi (brotli compression, min v1.2).

Changes Since 3.13.0

3.13.5 (2026-03-31): Skipped duplicate singleton header check in lax mode (default for response parsing).

3.13.4 (2026-03-28): Added max_headers parameter to limit headers read from a response. Added dns_cache_max_size parameter to TCPConnector. Fixed server hanging on mismatched chunked transfer encoding. Fixed access log DST timestamps. Fixed RuntimeError with GunicornWebWorker on Python >=3.14. Fixed TLS connection error with ClientTimeout(total=0). Restored synchronous BodyPartReader.decode() (new async decode_iter() available). Upgraded llhttp to 3.9.1.

3.13.3 (2026-01-03): Security release. Fixed proxy authorization headers not passing on connection reuse (407 errors). Fixed multipart reading with empty body parts. Fixed WebSocket continuation frame parser exception. Brotli/brotlicffi min version now 1.2 with 32MiB decompression limit. Moved dependency metadata to pyproject.toml (PEP 621).

3.13.2 (2025-10-28): Fixed cookie parser to continue on malformed cookies. Fixed netrc credential loading from default ~/.netrc. Fixed WebSocket compressed sends to be cancellation-safe.

3.13.1 (2025-10-17): AppRunner config options now available in run_app(). Switched to backports.zstd for Python <3.14. Fixed Content-Type parsing for invalid syntax. Fixed Python 3.14 support without zstd. Fixed blocking I/O in event loop with netrc auth. Fixed sub-application routing via .add_domain().

When to Use

• Building async web applications and REST APIs in Python
• Making concurrent HTTP requests from Python with connection pooling
• Implementing WebSocket servers or clients
• Creating high-performance microservices with asyncio
• Streaming large file uploads/downloads without loading into memory
• Building async middleware for request/response processing
• Testing aiohttp-based applications with built-in test utilities

Core Concepts

ClientSession: The primary client object. Reuse a single session per application — never create one per request. Sessions manage connection pools, cookies, and keep-alive.

import aiohttp
import asyncio

async def main():
    async with aiohttp.ClientSession() as session:
        async with session.get('http://python.org') as response:
            print("Status:", response.status)
            html = await response.text()

web.Application: The primary server object. Holds routes, middlewares, signals, and shared state via dict-like interface.

from aiohttp import web

async def handle(request):
    name = request.match_info.get('name', "Anonymous")
    return web.Response(text=f"Hello, {name}")

app = web.Application()
app.add_routes([web.get('/', handle), web.get('/{name}', handle)])

if __name__ == '__main__':
    web.run_app(app)

async/await pattern: All aiohttp operations are coroutines. Use async with for sessions and responses to ensure proper cleanup.

Connection pooling: Sessions maintain a connection pool internally. Connection reuse and keep-alive are enabled by default, improving performance for repeated requests to the same host.

Installation / Setup

Install via pip:

pip install aiohttp

For speedups (recommended):

pip install aiohttp[speedups]

This includes aiodns (faster DNS resolution) and Brotli (brotli compression support).

Enable Python development mode during development for stricter parsing and additional checks:

python -X dev your_app.py

Usage Examples

Client — Basic Requests

import aiohttp
import asyncio

async def main():
    async with aiohttp.ClientSession() as session:
        # GET request
        async with session.get('https://httpbin.org/get') as resp:
            print(resp.status)
            data = await resp.json()

        # POST with JSON body
        async with session.post('https://httpbin.org/post', json={'key': 'value'}) as resp:
            print(await resp.json())

        # POST with form data
        payload = {'key1': 'value1', 'key2': 'value2'}
        async with session.post('https://httpbin.org/post', data=payload) as resp:
            print(await resp.text())

asyncio.run(main())

Client — Query Parameters

params = {'key1': 'value1', 'key2': 'value2'}
async with session.get('https://httpbin.org/get', params=params) as resp:
    pass  # URL becomes https://httpbin.org/get?key1=value1&key2=value2

Client — Response Reading

# Text response
text = await resp.text()
text_custom_encoding = await resp.text(encoding='windows-1251')

# Binary response
data = await resp.read()

# JSON response
json_data = await resp.json()

# Streaming large responses
with open(filename, 'wb') as fd:
    async for chunk in resp.content.iter_chunked(chunk_size):
        fd.write(chunk)

Client — Authentication

from aiohttp import BasicAuth

auth = BasicAuth(login="user", password="pass")
async with aiohttp.ClientSession(auth=auth) as session:
    async with session.get("https://httpbin.org/basic-auth/user/pass") as resp:
        print(await resp.text())

Client — File Upload

# Simple file upload
files = {'file': open('report.xls', 'rb')}
await session.post(url, data=files)

# With explicit filename and content type
data = aiohttp.FormData()
data.add_field('file',
               open('report.xls', 'rb'),
               filename='report.xls',
               content_type='application/vnd.ms-excel')
await session.post(url, data=data)

# Streaming upload (large files)
with open('massive-body', 'rb') as f:
    await session.post('https://httpbin.org/post', data=f)

Server — Routing

from aiohttp import web

app = web.Application()

# Imperative style
app.router.add_get('/', handle_get)
app.router.add_post('/submit', handle_post)

# Route table (Flask-like decorators)
routes = web.RouteTableDef()

@routes.get('/get')
async def handle_get(request):
    ...

@routes.post('/post')
async def handle_post(request):
    ...

app.router.add_routes(routes)

# Variable routes
app.router.add_get(r'/{name:\d+}', handler)  # custom regex

# Named routes for URL building
@routes.get('/root', name='root')
async def handler(request):
    url = request.app.router['root'].url_for().with_query({"a": "b"})

Server — Class-Based Views

class MyView(web.View):
    async def get(self):
        return await get_resp(self.request)

    async def post(self):
        return await post_resp(self.request)

app.router.add_view('/path/to', MyView)

Server — JSON Response

async def handler(request):
    data = {'some': 'data'}
    return web.json_response(data)

Server — Form Handling

async def do_login(request):
    data = await request.post()
    login = data['login']
    password = data['password']

Server — WebSocket

async def websocket_handler(request):
    ws = web.WebSocketResponse()
    await ws.prepare(request)

    async for msg in ws:
        if msg.type == web.WSMsgType.TEXT:
            if msg.data == 'close':
                await ws.close()
            else:
                await ws.send_str(f"Echo: {msg.data}")
        elif msg.type == web.WSMsgType.ERROR:
            break
    return ws

Server — Static Files

app.add_routes([web.static('/static', '/path/to/static/folder')])

# With directory listing
web.static('/prefix', path_to_folder, show_index=True)

# With cache busting
web.static('/prefix', path_to_folder, append_version=True)

Server — Data Sharing

# Application-level storage (use AppKey for type safety)
db_key = web.AppKey("db", Database)
app[db_key] = database_instance

async def handler(request):
    db = request.app[db_key]
    # use db...

# Request-level storage
async def handler(request):
    request['processed'] = True

# Cross-application config lookup (nested apps)
async def handler(request):
    data = request.config_dict[my_key]  # searches parent apps too

Client — Middleware

async def auth_middleware(req: aiohttp.ClientRequest, handler: aiohttp.ClientHandlerType) -> aiohttp.ClientResponse:
    req.headers["Authorization"] = get_auth_header()
    return await handler(req)

async with aiohttp.ClientSession(middlewares=(auth_middleware,)) as session:
    async with session.get("https://example.com") as resp:
        ...

# Middleware chaining follows onion pattern:
# middleware1 (pre) -> middleware2 (pre) -> request -> middleware2 (post) -> middleware1 (post)

Client — Timeouts

from aiohttp import ClientTimeout

timeout = ClientTimeout(total=300, sock_connect=30)
async with aiohttp.ClientSession(timeout=timeout) as session:
    ...

Testing with pytest

from aiohttp import web

async def test_hello(aiohttp_client):
    app = web.Application()
    app.router.add_get('/', hello_handler)
    client = await aiohttp_client(app)
    resp = await client.get('/')
    assert resp.status == 200
    text = await resp.text()
    assert 'Hello, world' in text

Testing with unittest

from aiohttp.test_utils import AioHTTPTestCase
from aiohttp import web

class MyAppTestCase(AioHTTPTestCase):
    async def get_application(self):
        app = web.Application()
        app.router.add_get('/', self.hello)
        return app

    async def test_example(self):
        async with self.client.request("GET", "/") as resp:
            self.assertEqual(resp.status, 200)

Advanced Topics

Client API: Session configuration, connectors, SSL/proxy support, tracing, cookie management → Client Reference

Server API: Application runners, middlewares, signals, graceful shutdown, nested applications, deployment patterns → Server Reference

Streaming and Multipart: StreamReader API, multipart reader/writer, file streaming → Streaming and Multipart