- Are there plans for an @app.route decorator like in Flask?
- Does aiohttp have a concept like Flask's "blueprint" or Django's "app"?
- How do I create a route that matches urls with a given prefix?
- Where do I put my database connection so handlers can access it?
- How can middleware store data for web handlers to use?
- Can a handler receive incoming events from different sources in parallel?
- How do I programmatically close a WebSocket server-side?
- How do I make a request from a specific IP address?
- What is the API stability and deprecation policy?
- How do I enable gzip compression globally for my entire application?
- How do I manage a ClientSession within a web server?
- How do I access database connections from a subapplication?
- How do I perform operations in a request handler after sending the response?
- How do I make sure my custom middleware response will behave correctly?
- Why is creating a ClientSession outside of an event loop dangerous?
As of aiohttp 2.3, :class:`~aiohttp.web.RouteTableDef` provides an API
similar to Flask's @app.route. See
:ref:`aiohttp-web-alternative-routes-definition`.
Unlike Flask's @app.route, :class:`~aiohttp.web.RouteTableDef`
does not require an app in the module namespace (which often leads
to circular imports).
Instead, a :class:`~aiohttp.web.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)
If you're writing a large application, you may want to consider using :ref:`nested applications <aiohttp-web-nested-applications>`, which are similar to Flask's "blueprints" or Django's "apps".
See: :ref:`aiohttp-web-nested-applications`.
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.
:class:`aiohttp.web.Application` object supports the :class:`dict` interface and provides a place to store your database connections or any other resource you want to share between handlers.
db_key = web.AppKey("db_key", DB)
async def go(request):
db = request.app[db_key]
cursor = await db.cursor()
await cursor.execute('SELECT 42')
# ...
return web.Response(status=200, text='ok')
async def init_app():
app = Application()
db = await create_connection(user='user', password='123')
app[db_key] = db
app.router.add_get('/', go)
return app
Both :class:`aiohttp.web.Request` and :class:`aiohttp.web.Application` support the :class:`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.
Yes.
As an example, we may have two event sources:
- WebSocket for events from an end user
- 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 :meth:`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 = asyncio.create_task(
read_subscription(ws, request.app[redis_key]))
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_the_message(msg) # your function here
await ws.send_str(answer)
finally:
await redis.unsubscribe('channel:1')
Let's say we have an application with two endpoints:
/echoa WebSocket echo server that authenticates the user/logout_userthat, 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 :class:`aiohttp.web.Application` instance
and call :meth:`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_key][user_id].add(ws)
try:
async for msg in ws:
ws.send_str(msg.data)
finally:
request.app[websockets_key][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_key][user_id]
if not ws.closed]
# Watch out, this will keep us from returning 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()
app.router.add_route('GET', '/echo', echo_handler)
app.router.add_route('POST', '/logout', logout_handler)
app[websockets_key] = defaultdict(set)
web.run_app(app, host='localhost', port=8080)
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))
async with aiohttp.ClientSession(connector=conn) as session:
...
.. seealso:: :class:`aiohttp.TCPConnector` and ``local_addr`` parameter.
aiohttp follows strong Semantic Versioning (SemVer).
Obsolete attributes and methods are marked as deprecated in the documentation and raise :class:`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 :ref:`the changelog <aiohttp_changes>`.
It's impossible. Choosing what to compress and what not to compress 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?).
:class:`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 :class:`aiohttp.DummyCookieJar`. If you need separate cookies
for different http calls but process them in logical chains, use a single
:class:`aiohttp.TCPConnector` with separate
client sessions and connector_owner=False.
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_key] = mainapp[db_key]
mainapp.add_subapp("/prefix", subapp)
This can also be done from a :ref:`cleanup context<aiohttp-web-cleanup-ctx>`:
async def db_context(app: web.Application) -> AsyncIterator[None]:
async with create_db() as db:
mainapp[db_key] = mainapp[subapp_key][db_key] = db
yield
mainapp[subapp_key] = subapp
mainapp.add_subapp("/prefix", subapp)
mainapp.cleanup_ctx.append(db_context)
Middlewares can be written to handle post-response operations, but they run after every request. You can explicitly send the response by calling :meth:`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
request.app[db_key].inc_pong()
return resp
A :class:`aiohttp.web.Response` object must be returned. This is required by aiohttp web contracts, even though the response has already been sent.
Sometimes your middleware handlers might need to send a custom response. This is just fine as long as you always create a new :class:`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!')
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.
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()