FAQ

Are there plans for an @app.route decorator like in Flask?

As of aiohttp 2.3, RouteTableDef provides an API similar to Flask’s @app.route. See Alternative ways for registering routes.

Unlike Flask’s @app.route, RouteTableDef does not require an app in the module namespace (which often leads to circular imports).

Instead, a RouteTableDef is decoupled from an application instance:

routes = web.RouteTableDef()

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


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

app.router.add_routes(routes)

Does aiohttp have a concept like Flask’s “blueprint” or Django’s “app”?

If you’re writing a large application, you may want to consider using nested applications, which are similar to Flask’s “blueprints” or Django’s “apps”.

See: Nested applications.

How do I create a route that matches urls with a given prefix?

You can do something like the following:

app.router.add_route('*', '/path/to/{tail:.+}', sink_handler)

The first argument, *, matches any HTTP method (GET, POST, OPTIONS, etc). The second argument matches URLS with the desired prefix. The third argument is the handler function.

Where do I put my database connection so handlers can access it?

aiohttp.web.Application object supports the dict interface and provides a place to store your database connections or any other resource you want to share between handlers.

async def go(request):
    db = request.app['db']
    cursor = await db.cursor()
    await cursor.execute('SELECT 42')
    # ...
    return web.Response(status=200, text='ok')


async def init_app(loop):
    app = Application(loop=loop)
    db = await create_connection(user='user', password='123')
    app['db'] = db
    app.router.add_get('/', go)
    return app

Why is Python 3.5.3 the lowest supported version?

Python 3.5.2 fixes the protocol for async iterators: __aiter__() is not a coroutine but a regular function.

Python 3.5.3 has a more important change: asyncio.get_event_loop() returns the running loop instance if called from a coroutine. Previously it returned a default loop, set by asyncio.set_event_loop().

Previous to Python 3.5.3, asyncio.get_event_loop() was not reliable, so users were forced to explicitly pass the event loop instance everywhere. If a future object were created for one event loop (e.g. the default loop) but a coroutine was run by another loop, the coroutine was never awaited. As a result, the task would hang.

Keep in mind that every internal await expression either passed instantly or paused, waiting for a future.

It’s extremely important that all tasks (coroutine runners) and futures use the same event loop.

How can middleware store data for web handlers to use?

Both aiohttp.web.Request and aiohttp.web.Application support the dict interface.

Therefore, data may be stored inside a request object.

async def handler(request):
    request['unique_key'] = data

See https://github.com/aio-libs/aiohttp_session code for an example. The aiohttp_session.get_session(request) method uses SESSION_KEY for saving request-specific session information.

As of aiohttp 3.0, all response objects are dict-like structures as well.

Can a handler receive incoming events from different sources in parallel?

Yes.

As an example, we may have two event sources:

1. WebSocket for events from an end user
2. Redis PubSub for events from other parts of the application

The most native way to handle this is to create a separate task for PubSub handling.

Parallel aiohttp.web.WebSocketResponse.receive() calls are forbidden; a single task should perform WebSocket reading. However, other tasks may use the same WebSocket object for sending data to peers.

async def handler(request):

    ws = web.WebSocketResponse()
    await ws.prepare(request)
    task = request.app.loop.create_task(
        read_subscription(ws,
                          request.app['redis']))
    try:
        async for msg in ws:
            # handle incoming messages
            # use ws.send_str() to send data back
            ...

    finally:
        task.cancel()

async def read_subscription(ws, redis):
    channel, = await redis.subscribe('channel:1')

    try:
        async for msg in channel.iter():
            answer = process message(msg)
            ws.send_str(answer)
    finally:
        await redis.unsubscribe('channel:1')

How do I programmatically close a WebSocket server-side?

Let’s say we have an application with two endpoints:

1. /echo a WebSocket echo server that authenticates the user
2. /logout_user that, when invoked, closes all open WebSockets for that user.

One simple solution is to keep a shared registry of WebSocket responses for a user in the aiohttp.web.Application instance and call aiohttp.web.WebSocketResponse.close() on all of them in /logout_user handler:

async def echo_handler(request):

    ws = web.WebSocketResponse()
    user_id = authenticate_user(request)
    await ws.prepare(request)
    request.app['websockets'][user_id].add(ws)
    try:
        async for msg in ws:
            ws.send_str(msg.data)
    finally:
        request.app['websockets'][user_id].remove(ws)

    return ws


async def logout_handler(request):

    user_id = authenticate_user(request)

    ws_closers = [ws.close()
                  for ws in request.app['websockets'][user_id]
                  if not ws.closed]

    # Watch out, this will keep us from returing the response
    # until all are closed
    ws_closers and await asyncio.gather(*ws_closers)

    return web.Response(text='OK')


def main():
    loop = asyncio.get_event_loop()
    app = web.Application(loop=loop)
    app.router.add_route('GET', '/echo', echo_handler)
    app.router.add_route('POST', '/logout', logout_handler)
    app['websockets'] = defaultdict(set)
    web.run_app(app, host='localhost', port=8080)

How do I make a request from a specific IP address?

If your system has several IP interfaces, you may choose one which will be used used to bind a socket locally:

conn = aiohttp.TCPConnector(local_addr=('127.0.0.1', 0), loop=loop)
async with aiohttp.ClientSession(connector=conn) as session:
    ...

See also

aiohttp.TCPConnector and local_addr parameter.

What is the API stability and deprecation policy?

aiohttp follows strong Semantic Versioning (SemVer).

Obsolete attributes and methods are marked as deprecated in the documentation and raise DeprecationWarning upon usage.

Assume aiohttp X.Y.Z where X is major version, Y is minor version and Z is bugfix number.

For example, if the latest released version is aiohttp==3.0.6:

3.0.7 fixes some bugs but have no new features.

3.1.0 introduces new features and can deprecate some API but never remove it, also all bug fixes from previous release are merged.

4.0.0 removes all deprecations collected from 3.Y versions except deprecations from the last 3.Y release. These deprecations will be removed by 5.0.0.

Unfortunately we may have to break these rules when a security vulnerability is found. If a security problem cannot be fixed without breaking backward compatibility, a bugfix release may break compatibility. This is unlikely, but possible.

All backward incompatible changes are explicitly marked in the changelog.

How do I enable gzip compression globally for my entire application?

It’s impossible. Choosing what to compress and what not to compress is is a tricky matter.

If you need global compression, write a custom middleware. Or enable compression in NGINX (you are deploying aiohttp behind reverse proxy, right?).

How do I manage a ClientSession within a web server?

aiohttp.ClientSession should be created once for the lifetime of the server in order to benefit from connection pooling.

Sessions save cookies internally. If you don’t need cookie processing, use aiohttp.DummyCookieJar. If you need separate cookies for different http calls but process them in logical chains, use a single aiohttp.TCPConnector with separate client sessions and own_connector=False.

How do I access database connections from a subapplication?

Restricting access from subapplication to main (or outer) app is a deliberate choice.

A subapplication is an isolated unit by design. If you need to share a database object, do it explicitly:

subapp['db'] = mainapp['db']
mainapp.add_subapp('/prefix', subapp)

How do I perform operations in a request handler after sending the response?

Middlewares can be written to handle post-response operations, but they run after every request. You can explicitly send the response by calling aiohttp.web.Response.write_eof(), which starts sending before the handler returns, giving you a chance to execute follow-up operations:

def ping_handler(request):
    """Send PONG and increase DB counter."""

    # explicitly send the response
    resp = web.json_response({'message': 'PONG'})
    await resp.prepare(request)
    await resp.write_eof()

    # increase the pong count
    APP['db'].inc_pong()

    return resp

A aiohttp.web.Response object must be returned. This is required by aiohttp web contracts, even though the response already been sent.

How do I make sure my custom middleware response will behave correctly?

Sometimes your middleware handlers might need to send a custom response. This is just fine as long as you always create a new aiohttp.web.Response object when required.

The response object is a Finite State Machine. Once it has been dispatched by the server, it will reach its final state and cannot be used again.

The following middleware will make the server hang, once it serves the second response:

from aiohttp import web

def misbehaved_middleware():
    # don't do this!
    cached = web.Response(status=200, text='Hi, I am cached!')

    @web.middleware
    async def middleware(request, handler):
        # ignoring response for the sake of this example
        _res = handler(request)
        return cached

    return middleware

The rule of thumb is one request, one response.

Why is creating a ClientSession outside of an event loop dangerous?

Short answer is: life-cycle of all asyncio objects should be shorter than life-cycle of event loop.

Full explanation is longer. All asyncio object should be correctly finished/disconnected/closed before event loop shutdown. Otherwise user can get unexpected behavior. In the best case it is a warning about unclosed resource, in the worst case the program just hangs, awaiting for coroutine is never resumed etc.

Consider the following code from mod.py:

import aiohttp

session = aiohttp.ClientSession()

async def fetch(url):
    async with session.get(url) as resp:
        return await resp.text()

The session grabs current event loop instance and stores it in a private variable.

The main module imports the module and installs uvloop (an alternative fast event loop implementation).

main.py:

import asyncio
import uvloop
import mod

asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
asyncio.run(main())

The code is broken: session is bound to default asyncio loop on import time but the loop is changed after the import by set_event_loop(). As result fetch() call hangs.

To avoid import dependency hell aiohttp encourages creation of ClientSession from async function. The same policy works for web.Application too.

Another use case is unit test writing. Very many test libraries (aiohttp test tools first) creates a new loop instance for every test function execution. It’s done for sake of tests isolation. Otherwise pending activity (timers, network packets etc.) from previous test may interfere with current one producing very cryptic and unstable test failure.

Note: class variables are hidden globals actually. The following code has the same problem as mod.py example, session variable is the hidden global object:

class A:
    session = aiohttp.ClientSession()

    async def fetch(self, url):
        async with session.get(url) as resp:
            return await resp.text()